Background: The PI3K/protein kinase B (AKT) pathway is commonly activated in several tumor types. Selective targeting of p110β could result in successful pathway inhibition while avoiding the on- and off-target effects of pan-PI3K inhibitors. GSK2636771 is a potent, orally bioavailable, adenosine triphosphate-competitive, selective inhibitor of PI3Kβ.

Methods: We evaluated the safety, pharmacokinetics, pharmacodynamics and antitumor activity of GSK2636771 to define the recommended phase II dose (RP2D). During the dose-selection and dose-escalation stages (parts 1 and 2), patients with PTEN-deficient advanced solid tumors received escalating doses of GSK2636771 (25–500 mg once daily) using a modified 3+3 design to determine the RP2D; tumor type-specific expansion cohorts (part 3) were implemented to further assess tumor responses at the RP2D.

Results: A total of 65 patients were enrolled; dose-limiting toxicities were hypophosphatemia and hypocalcemia. Adverse events included diarrhea (48%), nausea (40%), and vomiting (31%). Single- and repeat-dose exposure increased generally dose proportionally. GSK2636771 400 mg once daily was the RP2D. Phospho/total AKT ratio decreased with GSK2636771 in tumor and surrogate tissue. A castrate-resistant prostate cancer (CRPC) patient harboring PIK3CB amplification had a partial response for over a year; an additional 10 patients derived durable (≥24 weeks) clinical benefit, including two other patients with CRPC with PIK3CB alterations (≥34 weeks). GSK2636771 400 mg once daily orally induced sufficient exposure and target inhibition with a manageable safety profile.

Conclusions: Genomic aberrations of PIK3CB may be associated with clinical benefit from GSK2636771. Clin Cancer Res; 23(19); 5981–92. ©2017 AACR.

Translational Relevance

The PI3K/AKT pathway is commonly activated in several tumor types. Selective targeting of p110β could result in successful pathway inhibition while avoiding the on- and off- target effects of pan-PI3K inhibitors. We present preclinical studies conducted to characterize the effect and define the optimal target population for development of GSK2636771, a selective p110β inhibitor, and results from a first-time-in-human clinical trial study indicating that GSK2636771 can be active in PTEN-deficient and/or PIK3CB-aberrant advanced solid tumors. Further phase II clinical studies of GSK2636771 in combination with other agents, including androgen receptor antagonists, are currently underway based on these data.

Activation of PI3K/protein kinase B (AKT)/mTOR signaling (1), most commonly by activating mutations of PI3K/AKT family members or loss of PTEN phosphatase function, contributes to carcinogenesis of many malignancies (2–5). Inhibition of PI3K signaling has been challenging therapeutically, and inhibitors of PI3K have so far had limited clinical success (6, 7). Reasons for this include biological feedback loops permitting the tumor to reactivate the pathway (8, 9), activation of alternative pathways (10), and the nonspecificity of pan-PI3K inhibitors, resulting in a plethora of PI3K-related and off-target toxicities that limit administration of clinically active doses for long enough (11). PI3K is composed of a heterodimer between a p110 catalytic subunit and a p85 regulatory subunit. The four described isoforms of the catalytic subunit are p110α, p110β, p110γ, and p110δ, encoded by genes PIK3CA, PIK3CB, PIK3CG, and PIK3CD, respectively (12). PI3K-isoform-selective inhibitors have been developed in attempts to reduce off-target toxicities seen with pan-PI3K inhibitors (13).

Loss of PTEN function is common in a number of cancers, including glioblastoma, prostate, endometrial, melanoma, and breast cancers. Preclinical studies have indicated that the PI3Kβ isoform (containing the p110β catalytic subunit) is the critical lipid kinase that drives primarily PI3K pathway activation, cell growth, and survival in PTEN-deficient tumor cells (14–17).

We hypothesize that highly selective PI3Kβ inhibition would have utility in PTEN-deficient cancers, while avoiding toxicities associated with inhibition of other PI3K isoforms (18), in particular skin rash, hyperglycemia and diarrhea, or other off-target effects. This strategy would be expected to maximize therapeutic efficacy by enabling administration of appropriate doses and rational drug combinations with other agents, such as androgen receptor antagonists in PTEN-deficient prostate cancer (19–21), or erbB2 inhibitors and hormonal treatments in breast cancer (22–25).

GSK2636771 (Fig. 1A) is a potent, orally bioavailable, adenosine triphosphate-competitive, selective inhibitor of PI3Kβ with an apparent Ki value of 0.89 nmol/L (IC50 = 5.2 nmol/L), >900-fold selectivity over p110α and p110γ, and >10-fold selectivity over p110δ isoforms, while sparing other PI3K superfamily kinases (Fig. 1B). Although pan-PI3K inhibitors have been tested in clinical trials, to our knowledge, this is the first study of a truly selective inhibitor of PI3Kβ, which confers the advantage of avoiding on- and off-target toxicities associated with pan-PI3K inhibitors.

Figure 1.

GSK2636771 is a potent, selective inhibitor of PI3Kβ that exhibits antitumor activity in PTEN-deficient cancers. A, Chemical structure of GSK2636771. B, The selectivity of GSK2636771 for PI3Kβ against other PI3K isoforms and PI3K family members was tested in a biochemical activity assay (left column) and using a cancer cell lysate-based chemoproteomics approach to measure binding affinity (right column). C, Anchorage independent tumor cell growth was measured after 6 days of GSK2636771 treatment (dose range, 0.16–3.07 μmol/L) comparing PTEN WT with PTEN-deficient cells. Error bars correspond to SD. D, HCC1954 and MDA-MB-468 breast cancer cells were treated with increasing concentrations of GSK2636771 for 24 hours, and lysates were probed by Western blot analysis using the indicated antibodies. E, PC3 prostate cancer cells were treated with GSK2636771 1 or 10 μmol/L for up to 48 hours and probed with the indicated antibodies. F, Mice bearing subcutaneous PC3 tumor xenografts (n = 8/group) were treated with vehicle or GSK2636771 (1, 3, or 10 mg/kg) once daily by oral gavage for 21 days, and tumor volumes were assessed. Error bars correspond to standard error of the mean. G, PK/PD relationship of GSK2636771 was tested in mice bearing PC3 tumor xenografts (n = 3/group) dosed once orally with either vehicle, 3 mg/kg or 10 mg/kg of GSK2636771. Tumors and blood samples were harvested at the indicated time points to measure plasma compound concentration and the ratio of pAKT to total AKT in tumors using enzyme-linked immunosorbent assays. The numbers above the bars indicate percent inhibition of pAKT relative to vehicle-treated tumors. Error bars correspond to SD. PD, pharmacodynamics; PK, pharmacokinetic.

Figure 1.

GSK2636771 is a potent, selective inhibitor of PI3Kβ that exhibits antitumor activity in PTEN-deficient cancers. A, Chemical structure of GSK2636771. B, The selectivity of GSK2636771 for PI3Kβ against other PI3K isoforms and PI3K family members was tested in a biochemical activity assay (left column) and using a cancer cell lysate-based chemoproteomics approach to measure binding affinity (right column). C, Anchorage independent tumor cell growth was measured after 6 days of GSK2636771 treatment (dose range, 0.16–3.07 μmol/L) comparing PTEN WT with PTEN-deficient cells. Error bars correspond to SD. D, HCC1954 and MDA-MB-468 breast cancer cells were treated with increasing concentrations of GSK2636771 for 24 hours, and lysates were probed by Western blot analysis using the indicated antibodies. E, PC3 prostate cancer cells were treated with GSK2636771 1 or 10 μmol/L for up to 48 hours and probed with the indicated antibodies. F, Mice bearing subcutaneous PC3 tumor xenografts (n = 8/group) were treated with vehicle or GSK2636771 (1, 3, or 10 mg/kg) once daily by oral gavage for 21 days, and tumor volumes were assessed. Error bars correspond to standard error of the mean. G, PK/PD relationship of GSK2636771 was tested in mice bearing PC3 tumor xenografts (n = 3/group) dosed once orally with either vehicle, 3 mg/kg or 10 mg/kg of GSK2636771. Tumors and blood samples were harvested at the indicated time points to measure plasma compound concentration and the ratio of pAKT to total AKT in tumors using enzyme-linked immunosorbent assays. The numbers above the bars indicate percent inhibition of pAKT relative to vehicle-treated tumors. Error bars correspond to SD. PD, pharmacodynamics; PK, pharmacokinetic.

Close modal

Here we present preclinical data characterizing the selectivity of GSK2636771 in cell line and murine xenograft models, together with the results of a dose-finding, first-time-in-human study of GSK2636771 monotherapy in patients with PTEN-deficient or PIK3CB genomically altered advanced solid tumors. The aim of this first-time-in-human study was to further characterize the tolerability, safety and pharmacokinetic–pharmacodynamic (PK–PD) profile of GSK2636771, while also assessing its antitumor activity. We also pursued genomics analyses to identify any alterations as putative predictive biomarkers of antitumor response to determine the optimal target population.

Preclinical studies

Cell lines and reagents.

Cell lines were obtained from ATCC, cultured in the appropriate medium supplemented with 10% FBS (Sigma−Aldrich) at 37 °C in humidified incubators under 5% carbon dioxide, and passaged no greater than 20 times. The cell lines were authenticated by short tandem repeat (STR) profiling and tested for Mycoplasma upon receipt using the ATCC Universal Mycoplasma Detection Kit. GSK2636771 was dissolved in dimethyl sulfoxide at a stock concentration of 20 mmol/L.

Selectivity of GSK2636771 for PI3Kβ.

Biochemical selectivity of GSK2636771 was tested using the PI3-Kinase HTRF Assay (EMD Millipore), as well as the entire panel of GSK in-house kinase selectivity assays. Affinity-enrichment-based chemoproteomics using kinobeads was performed as described previously (26). Briefly, 14 lipid and atypical kinases were enriched from a standard mixture of extracts derived from HeLa, K562, and Jurkat cells using a compound-derivatized bead matrix. The enriched proteins were identified by quantitative mass spectrometry analysis, enabling the simultaneous assessment of binding specificity, and potency for all detected affinity-captured proteins.

Soft agar cell-viability assay.

Cells were cultured in 96-well plates (5 × 103 cells/well) and treated with GSK2636771 (dose range, 30.7 μmol/L–1.6 nmol/L) for 6 days in soft agar media (bottom layer: 0.6% final concentration; top layer: 0.3% final concentration). Cell proliferation was measured using the alamarBlue Cell Viability Assay (Thermo Fisher) according to the manufacturer's instructions. One cell plate was developed with alamarBlue reagent at the time of compound addition (T0 plate). Results were then expressed as a percentage of the T0 value (normalized to 100%) and plotted against the compound concentration after 6 days of treatment. The cellular response was determined by fitting the concentration response data using a four-parameter curve fit equation and determining the concentration that inhibited 50% of the YmaxYmin window (EC50).

Western blot analysis.

HCC1954 and MDA-MB-468 breast cancer cells were treated with increasing concentrations of GSK2636771 for 24 hours. PC3 prostate cancer cells were treated with 1 or 10 μmol/L GSK2636771 for up to 48 hours. Cells were lysed with 1× cell lysis buffer (Cell Signaling Technology) containing protease and phosphatase inhibitors (Roche). Subsequently, 30 to 40 μg of protein was run on 4% to 12% Bis-Tris gels (Thermo Fisher), and protein was transferred onto nitrocellulose membranes (Thermo Fisher). Membranes were blocked for 1 hour using Odyssey Blocking Buffer (LI-COR Biosciences), before immunoblotting using the following antibodies (all from Cell Signaling Technology): pAKT Ser473 (#4060), pAKT Thr308 (#13038), total AKT (#9272), pERK (#9101), total ERK (#4695), pS6 (#2211), total S6 (#2317), PTEN (#9188), and p100β (#3011). Western blots were processed using Odyssey CLx Imaging System (LI-COR Biosciences).

In vivo studies.

Female nude mice (Charles River Laboratories) were injected with 2.0 × 106 PC3 cells to establish subcutaneous PC3 tumor xenografts. Once tumors reached ∼200 to 250 mm3, mice were randomized (n = 8/group) and treated with vehicle or GSK2636771 at 1, 3, 10, or 30 mg/kg by oral gavage for 21 days. Tumor volume measurements and body weights were collected twice weekly. For PK/PD studies, mice bearing PC3 tumor xenografts (n = 3/group) were dosed once orally with either vehicle or GSK2636771 at 3 and 10 mg/kg for 1, 2, 4, 6, 8, 10, and 24 hours. Blood was collected and mixed 1:1 with water, and tumors were excised into two halves with one half flash frozen in liquid nitrogen for compound concentration determination by the GSK Drug Metabolism and PK (DMPK) group. The other half of excised tumors was immediately processed using a sterile Medicon (BD Biosciences) in 1 mL Meso-Scale Discovery (MSD) lysis buffer containing protease and phosphatase inhibitors. Phospho and total AKT protein levels were measured using the MSD Phospho (Ser473)/Total AKT Whole Cell Lysate ELISA Kit according to the manufacturer's instructions. To measure glucose and insulin response, female nude mice (n = 3/group) were dosed orally for 3 days with vehicle, 100 mg/kg GSK2636771, or 3 mg/kg GSK2126458 (a pan PI3K/mTOR inhibitor), then starved for 20 hours before receiving a final dose of compound followed by blood collection after 0, 0.5, 1, 2, and 4 hours. Compound concentrations were determined by the GSK DMPK group, glucose was measured using an ACCU-CHEK Compact Plus glucose meter (Roche), and insulin was measured from plasma using an ALPCO Mouse Insulin ELISA Kit. All animal studies were conducted in accordance with the GSK Policy on the Care, Welfare and Treatment of Laboratory Animals and were reviewed by the Institutional Animal Care and Use Committee at GSK.

First-time-in-human study

Study design.

The study followed a multistage design to maximize the number of patients receiving potentially active doses and prioritize acquisition of tumor tissue biopsies for PD analysis (Supplementary Fig. S1). Part 1 was a dose selection stage, to assess the PK of GSK2636771 following single-dose administration and determine the optimal starting dose for part 2. The primary objective of part 1 was to establish a GSK2636771 dose that provided a median area under the concentration–time curve 0 to 24 hours (AUC[0–24]) at steady state of 10–50 μg·h/mL. Part 2 was a dose-escalation stage utilizing a modified 3+3 design and allowing enrollment of additional patients for PD analysis of tumor biopsies. The primary objectives of part 2 were to determine a recommended phase II dose (RP2D), further characterize the PK and PD of GSK2636771 after repeated daily dosing, and confirm the inhibition of PI3Kβ activity by GSK2636771 in tumor biopsies. Part 3 was an expansion cohort stage including patients with PTEN-deficient tumors and/or genomic PIK3CB genomic aberrations, to determine tumor responses to the RP2D of GSK2636771.

Clinical trial oversight.

The study was designed by GSK representatives and study investigators. The research ethics committee at each participating site approved the study protocol. Data were collated and analyzed by GSK.

Trial population.

Patients with advanced solid tumors progressing on standard therapy were enrolled after providing written consent and based on eligibility criteria. These included: age ≥18 years; Eastern Cooperative Oncology Group performance status 0 to 1; adequate organ function including renal function (based on blood creatinine and urine protein/creatinine ratio); and normal left-ventricular ejection fraction (LVEF). Patients receiving medication impacting platelet aggregation or with a baseline platelet-function defect were excluded. Full eligibility criteria can be found in the Supplementary Appendix.

For parts 1 and 2, the target population were patients with PTEN-deficient tumors (determined by IHC) and one of the following primary tumor types: endometrial, ovarian, triple-negative breast cancer, castrate-resistant prostate cancer (CRPC), non–small cell lung cancer, glioblastoma, gastric adenocarcinoma, colorectal, head and neck squamous carcinoma, and melanoma. In part 3, the expansion cohorts included patients with PTEN-deficient CRPC, colorectal cancer, and/or genomic abnormalities (copy-number gain or mutations) in PIK3CB. For eligibility purposes, PTEN assessments during the dose-escalation stage of the trial were performed at either the local laboratory of the investigator sites or at a central laboratory (Ventana Medical Systems). In the expansion phase of the study, all samples were tested at the central laboratory (Ventana Medical Systems) prior to enrollment and loss of PTEN function was defined as an H-score ≤30, with a maximum of 30% of cells at a 1+ staining intensity. A rabbit monoclonal anti-PTEN antibody (clone D4.3, catalog no. 9188, Cell Signaling Technologies) was used for PTEN IHC staining.

Treatment, starting dose, and dose-escalation.

Treatment was administered orally as white gelatin capsules containing 10, 25, or 100 mg of GSK2636771. Based on nonclinical toxicology studies predicting an AUC[0–24] in human subjects of 13 μg·h/mL and a maximum observed plasma concentration (Cmax) of 0.85 μg/mL at steady state, the starting dose in part 1 was 25 mg. This was <1/20 of the dose estimated with FDA recommendations for starting doses based on the highest nonseverely toxic dose (HNSTD) of 100 mg/kg/day, which was also the no observed adverse effect level (NOAEL) in canine studies. Part 2 followed a modified 3+3 design (Supplementary Table S1), starting at the selected dose from part 1. Dose-limiting toxicities (DLT) were defined as any grade 3/4 nonhematological drug-related toxicity (apart from grade 3 rash, diarrhea, nausea, vomiting, or mucositis that responds to treatment within 48 hours) occurring during the first 4 weeks of drug administration. In addition, grade 4 neutropenia lasting >5 days, grade 4 anemia, grade 4 thrombocytopenia (or grade 3 with bleeding), an eight-fold increase in transaminases (over the upper limit of normal), a >20% decrease in LVEF, or any toxicity leading to >25% of the planned dose being missed, were also considered DLTs. Dose escalation was pursued until the maximum tolerated dose was established, defined as the maximum dose level before DLTs were observed in ≥33% of patients.

Study evaluations.

Adverse events (AEs) were recorded throughout the study, and graded based on Common Terminology Criteria for Adverse Events v4.0, including monitoring of changes in renal function via blood and urine tests and other vital signs assessments. Cardiac evaluations (echocardiograms/multigated acquisition scans) were performed at baseline and bimonthly during treatment. Response to therapy was assessed every 8 weeks by computed tomography/magnetic resonance imagery (and whole-body bone scintigraphy for patients with CRPC) (27). Tumor markers were analyzed every 8 weeks if appropriate, according to tumor type.

Blood samples for PK analysis were collected ≤1 hour pre-dose and 0.5, 1, 2, 3, 4, 6, 8, 10, 24, 48, and 72 hours after single dose administration (parts 1 and 2) and then ≤1 hour pre-dose on days 8 and 15 and ≤1 hour pre-dose and 0.5, 1, 2, 3, 4, 6, 8, 10, and 24 hours after administration on day 22 during the first cycle of continuous treatment (part 2). Blood samples at ≤1 hour pre-dose, 1 to 2 hours, 3 to 4 hours, 6 to 8 hours, and 22 to 26 hours post-dose on day 22 were collected in part 3.

Analyses of markers of target modulation (pSer473 AKT, pSer9 GSK3β, and pThr421/Ser424 P70S6K) were undertaken on platelet-rich plasma (PRP) from patients during the dose-escalation stage using MSDelectrochemiluminescent immunoassays validated to Good Clinical Practice standards. Changes in pSer473 AKT, pThr246 PRAS40, pSer235/236 S6RP, and pThr308 AKT were measured in tumor biopsies using IHC (H-scores) at pretreatment and days 8 to 15 (2–4 hours post-dose).

Next-generation sequencing and copy-number analyses.

Retrospective targeted next-generation sequencing of archival or fresh tumor samples was performed if tissue was available. DNA was extracted using the GeneRead formalin-fixed, paraffin-embedded DNA Isolation Kit (Qiagen; Catalog No. 180134) and libraries prepared utilizing a customized sequencing panel (Qiagen GeneRead v2; Supplementary Table S2) including PI3K/AKT pathway genes, and sequencing was carried out on an Illumina Sequencer. Copy-number variation was determined using Nanostring or quantitative polymerase chain reaction platforms. Background corrected, normalized values relative to a normal (diploid) control for one to three probes were used for each gene.

Functional characterization of the PIK3CB p.L1049R mutation in vitro

BacMam vectors (pHTBV1.1) containing human wild-type (WT) p110β or mutant p110β (L1049R) were obtained from the GSK plasmid repository. Viral particles were generated and added into six-well plates at a range of 0 to 500 multiplicity of infection. PC3 cells were then plated in the wells (1.0 × 106 cells/well) and allowed to incubate overnight. The media containing viral particles was removed and replaced with media lacking serum for 10 hours. PC3 cells were then lysed for Western blot analysis.

Statistical considerations

The number of subjects in parts 1 and 2 was dependent on the number of subjects enrolled to select a starting dose, characterize individual cohorts, and explore the PD profile. Planned enrolment for part 3 included a minimum of 12 and maximum of 20 subjects in each Tumor-Specific Expansion Cohort.

Descriptive statistics were used to summarize safety data in all patients who received at least one dose of GSK2636771. All patients who underwent sampling were included in the PK analyses, which used descriptive statistics to summarize AUC(0–t), AUC(0–24), Cmax, time to reach Cmax (Tmax), calculated using standard noncompartmental methods. In addition, AUC(0–∞) and half-life were assessed after the single run-in dose. Tumor response rate was evaluated according to Response Evaluation Criteria In Solid Tumors (RECIST) 1.1 criteria (28). The data were analyzed with Statistical Analysis Software (SAS) v9.2.

Preclinical studies

To determine the effects of selectively inhibiting PI3Kβ activity on tumor cell growth and pathway signaling, a series of experiments was performed with GSK2636771 to compare its effects in PTEN-deficient and PTEN WT tumor cells. GSK2636771 primarily inhibited the growth of PTEN-deficient cancer cells in a cell line panel spanning multiple tumor types (Fig. 1C). Inhibition of AKT and ribosomal S6 kinase phosphorylation was observed in a concentration- and time-dependent manner mainly in PTEN-deficient cells (Fig. 1D and E and Supplementary Fig. S2). GSK2636771 had no effect on MAPK signaling, as evidenced by measurement of ERK phosphorylation (Fig. 1D). When administered orally in mice bearing PC-3 prostate tumor xenografts, GSK2636771 resulted in tumor growth inhibition, and a dose- and time-dependent PK–PD response was observed (Fig. 1F and G). Importantly, GSK2636771 did not elevate glucose or insulin levels in mice compared with the pan PI3K/mTOR inhibitor, GSK2126458 (Supplementary Fig. S3).

First-time-in-human clinical study

Patients and administered treatments.

Overall, 65 patients were enrolled and received at least one dose of study medication. Data from three patients in part 1 (dose selection), 50 in part 2 (dose-escalation and additional PD exploratory cohorts), and 10 as part of the expansion cohorts in part 3 made up a cohort of 63 patients. Data from the last two patients with known PIK3CB genomic alterations made up the overall trial population of 65, but were analyzed separately for some parameters and were not included in the PK–PD analyses. Baseline patient demographics and characteristics for the entire population are summarized in Table 1. Briefly, the median age of the study population was 62 years (range, 30–79), 26 (40%) patients were female, and the most common tumor types were colorectal (n = 23, 35%) and prostate cancers (n = 12, 18%). All patients had received at least one previous anticancer treatment; 37 (57%) had received >4 anticancer treatments. In total, seven dose levels (25–500 mg once daily) were investigated. Median time on treatment was 55 days (range, 5–478).

Table 1.

Baseline patient demographics and clinical characteristics

Total (N = 65)
Age, years 
 Mean (SD) 59.7 (11.13) 
 Median (range) 62.0 (30–79) 
 Female, n (%) 26 (40) 
Race, n (%) 
 African American/African 2 (3) 
 Asian (Central/Southern) 1 (2) 
 Asian (Eastern) 13 (20) 
 Caucasian 48 (74) 
 Unknown 1 (2) 
Primary tumor type, n (%) 
 Colon/rectum 23 (35) 
 Prostate 12 (18) 
 Gastric/GE junction 7 (11) 
 Breast (triple negative) 6 (9) 
 Ovary/fallopian tube 5 (8) 
 Endometrium/uterus 3 (5) 
 NSCLC 3 (5) 
 CNS 2 (3) 
 Head and neck 2 (3) 
 Melanoma 1 (2) 
 Cervix 1 (2) 
ECOG status, n (%) 
 0 37 (57) 
 1 28 (43) 
Prior lines of therapy, n (%) 
 1 7 (11) 
 2 9 (14) 
 3 4 (6) 
 4 8 (12) 
 >4 37 (57) 
Total (N = 65)
Age, years 
 Mean (SD) 59.7 (11.13) 
 Median (range) 62.0 (30–79) 
 Female, n (%) 26 (40) 
Race, n (%) 
 African American/African 2 (3) 
 Asian (Central/Southern) 1 (2) 
 Asian (Eastern) 13 (20) 
 Caucasian 48 (74) 
 Unknown 1 (2) 
Primary tumor type, n (%) 
 Colon/rectum 23 (35) 
 Prostate 12 (18) 
 Gastric/GE junction 7 (11) 
 Breast (triple negative) 6 (9) 
 Ovary/fallopian tube 5 (8) 
 Endometrium/uterus 3 (5) 
 NSCLC 3 (5) 
 CNS 2 (3) 
 Head and neck 2 (3) 
 Melanoma 1 (2) 
 Cervix 1 (2) 
ECOG status, n (%) 
 0 37 (57) 
 1 28 (43) 
Prior lines of therapy, n (%) 
 1 7 (11) 
 2 9 (14) 
 3 4 (6) 
 4 8 (12) 
 >4 37 (57) 

CNS, central nervous system; ECOG, Eastern Cooperative Oncology Group; GE, gastroesophageal junction; NSCLC, non–small cell lung cancer.

Selection of the starting dose for dose-escalation stage.

Three patients were enrolled in part 1 of the study and received a single dose of 25 mg of GSK2636771. The geometric mean AUC[0–24] was 15.7 μg · h/mL, which was within the prespecified target range of 10 to 50 μg · h/mL. Consequently, 25 mg once daily was selected as the initial dose for the dose-escalation stage.

Safety, tolerability, and DLTs.

No DLTs were observed in any patient receiving 25 to 350 mg once daily of GSK2636771. Dose-escalation then continued to 500 mg once daily, where three of four treated patients experienced a DLT [hypocalcemia (grades 2 and 3) and hypophosphatemia (grade 3)], during the first to third week of continuous treatment. One patient also experienced a grade 1 creatinine elevation. These toxicities, indicative of potential renal tubular damage, resolved after GSK2636771 discontinuation (except for one with normalized phosphate levels but persisting grade 1 hypocalcemia); two of the three patients were able to continue GSK2636771 treatment at a lower dose. An intermediate lower dose of 400 mg once daily was explored (n = 6), and no DLTs were observed. As such, 400 mg once daily was selected as the RP2D.

All 65 patients experienced at least one AE during the study; 60 patients experienced AEs that were considered to be related to the study drug (Table 2). The most common AEs across all dose levels (any; treatment-related) were gastrointestinal [diarrhea (n = 31, 48%; n = 27, 42%), nausea (n = 27, 41%; n = 18, 28%), vomiting (n = 20, 31%; n = 12, 18%)], and fatigue [n = 17 (26%), n = 14, 22%; Table 2]. AEs by dose level are shown in Supplementary Table S3. Overall, 10 (15%) patients had treatment permanently discontinued due to an AE. Reasons for discontinuation were fatigue (n = 1 in the 25 mg dose escalation cohort); fatigue/nausea (n = 1) and hypophosphatemia/hypocalcemia (n = 1), both in the 500 mg dose escalation cohort; proteinuria (n = 1; 200 mg); pain (n = 1), increased N-terminal prohormone brain natriuretic peptide/upper respiratory tract infection (n = 1) and small intestinal obstruction (n = 1), all with 350 mg of GSK2636771; and dyspnea (n = 1), fatigue/vomiting (n = 1), and urinary retention (n = 1), all in the 400 mg expansion cohort. Dose reductions were required during trial treatment by four of 18 (22%) patients treated at the RP2D of 400 mg once daily. Forty-one serious AEs (SAEs) occurred in 24 (37%) patients. Nine SAEs in five patients were considered related to the study drug (nausea, fatigue, hypocalcemia, and hypophosphatemia in one patient, increased creatinine and decreased appetite in one patient, and vomiting, urinary retention, and pruritic rash in one patient each). Apart from the grade 1 increased creatinine, related SAEs were grade 2 or 3. One patient experienced dyspnea, which had a fatal outcome, but the event was considered not related to study treatment. The primary cause of death for this patient was disease under study. Eight (12%) additional deaths occurred during the study, all of which were considered related to the underlying disease.

Table 2.

Summary of AEs and treatment-related AEs occurring in ≥10% of all patients, by grade

Any AETreatment-related AE
Total (N = 65)Total (N = 65)
Preferred term, n (%)Any gradeGrade ≥3Any gradeGrade ≥3
Any AE 65 (100) 33 (51) 60 (92) 15 (23) 
 Hypophosphatemia 8 (12) 6 (9) 6 (9) 5 (7) 
 Rash 10 (15) 3 (5) 5 (8) 3 (5) 
 Fatigue 17 (26) 3 (5) 14 (22) 2 (3) 
 Hypocalcemia 9 (14) 2 (3) 6 (9) 2 (3) 
 Diarrhea 31 (48) 1 (2) 27 (42) 1 (2) 
 Nausea 27 (41) 2 (3) 18 (28) 1 (2) 
 Vomiting 20 (31) 2 (3) 12 (18) 1 (2) 
 Decreased appetite 15 (23) 1 (2) 9 (14) 1 (2) 
 Increased aspartate aminotransferase 8 (12) 2 (3) 5 (8) 1 (2) 
 Hypertension 6 (9) 2 (3) 3 (5) 1 (2) 
 Rash maculopapular 3 (5) 1 (2) 3 (5) 1 (2) 
 Rash pruritic 3 (5) 1 (2) 3 (5) 1 (2) 
 Hypoalbuminemia 4 (6) 2 (3) 2 (3) 1 (2) 
 Increased NT-proBNP 3 (5) 1 (2) 2 (3) 1 (2) 
 Proteinuria 4 (6) 1 (2) 2 (3) 1 (2) 
 Dehydration 3 (5) 3 (5) 1 (2) 1 (2) 
 Nephropathy 1 (2) 1 (2) 1 (2) 1 (2) 
 Urinary retention 2 (3) 1 (2) 1 (2) 1 (2) 
 Dysgeusia 4 (6) 0 (0) 4 (6) 0 (0) 
 Headache 13 (20) 1 (2) 4 (6) 0 (0) 
 Increased blood creatinine 9 (14) 0 (0) 4 (6) 0 (0) 
Any AETreatment-related AE
Total (N = 65)Total (N = 65)
Preferred term, n (%)Any gradeGrade ≥3Any gradeGrade ≥3
Any AE 65 (100) 33 (51) 60 (92) 15 (23) 
 Hypophosphatemia 8 (12) 6 (9) 6 (9) 5 (7) 
 Rash 10 (15) 3 (5) 5 (8) 3 (5) 
 Fatigue 17 (26) 3 (5) 14 (22) 2 (3) 
 Hypocalcemia 9 (14) 2 (3) 6 (9) 2 (3) 
 Diarrhea 31 (48) 1 (2) 27 (42) 1 (2) 
 Nausea 27 (41) 2 (3) 18 (28) 1 (2) 
 Vomiting 20 (31) 2 (3) 12 (18) 1 (2) 
 Decreased appetite 15 (23) 1 (2) 9 (14) 1 (2) 
 Increased aspartate aminotransferase 8 (12) 2 (3) 5 (8) 1 (2) 
 Hypertension 6 (9) 2 (3) 3 (5) 1 (2) 
 Rash maculopapular 3 (5) 1 (2) 3 (5) 1 (2) 
 Rash pruritic 3 (5) 1 (2) 3 (5) 1 (2) 
 Hypoalbuminemia 4 (6) 2 (3) 2 (3) 1 (2) 
 Increased NT-proBNP 3 (5) 1 (2) 2 (3) 1 (2) 
 Proteinuria 4 (6) 1 (2) 2 (3) 1 (2) 
 Dehydration 3 (5) 3 (5) 1 (2) 1 (2) 
 Nephropathy 1 (2) 1 (2) 1 (2) 1 (2) 
 Urinary retention 2 (3) 1 (2) 1 (2) 1 (2) 
 Dysgeusia 4 (6) 0 (0) 4 (6) 0 (0) 
 Headache 13 (20) 1 (2) 4 (6) 0 (0) 
 Increased blood creatinine 9 (14) 0 (0) 4 (6) 0 (0) 

NT-proBNP, N-terminal prohormone brain natriuretic peptide.

Hyperglycemia, which has been reported when targeting other nodes in the PI3K/AKT/mTOR pathway (29), was reported in 36 (55%) patients receiving GSK2636771 treatment and was predominantly mild in severity; only two (3%) incidences of hyperglycemia were reported as AEs (grade 1 in one patient and grade 2 in the other). No cases of drug-related grade ≥3 hyperglycemia were observed in this study. Cutaneous toxicity was uncommon: 10 cases (15%) of skin rash were documented across all dose levels (four of which were in the GSK2636771 400 mg once-daily group). In addition, three cases of pruritic rash (200 and 500 mg once-daily dose-escalation cohorts and 400 mg once-daily expansion cohort) and three cases of maculopapular rash (50 mg once-daily PD, 350 mg once-daily dose escalation, and 400 mg once-daily expansion cohorts) were reported. Other than the aforementioned cases of hypophosphatemia and hypocalcemia, evidence for renal tubular toxicity also included proteinuria, which was reported in four (6%) patients: one in each of the GSK2636771 50, 200, 350, and 500 mg once-daily dose groups.

PK–PD.

PK–PD data are reported for the N = 63 cohort. Following a single run-in dose of GSK2636771, drug exposure (Cmax, AUC) increased dose proportionally up to 350 mg, with below-proportional increments above this dose. The median Tmax was 4 hours (range, 1–10 hours). Blood concentrations declined in a monophasic manner with a geometric mean half-life between 13 and 23 hours. Single-dose PK data are shown in Supplementary Tables S4 and S5. Similar dose-proportional findings [for both AUC(0–τ) and Cmax] were observed after repeated daily oral dosing at day 22, with ratio of AUC(0–∞) versus AUC(0–τ), suggesting steady-state had been achieved (Table 3 and Supplementary Table S6). PK parameters for the PD cohorts and concentrations in patients (n = 2) in part 3 were similar to those observed in the dose-escalation cohorts.

Table 3.

PK parameters following repeated daily oral dosing of GSK2636771 (PK population)

Dose selection cohortDose escalation cohort
25 mg25 mg50 mg100 mg200 mg350 mg400 mg500 mg
PK parameter, n (%)n = 3n = 6n = 4n = 3n = 3n = 7n = 6n = 4
Cmax, ng/mLa n = 2 n = 5 n = 4 n = 3 n = 3 n = 4 n = 6 n = 2 
 1,459 (13) 1,770 (65) 2,336 (43) 2,882 (85) 13,175 (6) 16,452 (9) 15,078 (55) 29,530 (71) 
Tmax, hour, median (range) n = 2 n = 5 n = 4 n = 3 n = 3 n = 4 n = 6 n = 2 
 8.96 (8.03, 9.88) 4.05 (3.00, 6.07) 4.05 (4.03, 6.00) 3.25 (3.10, 8.02) 23.9 (3.22, 23.92) 3.60 (2.03, 8.85) 2.01 (1.02, 5.80) 6.33 (2.58, 10.08) 
AUC(0–τ), hour·ng/mLa n = 2 n = 4 n = 4 n = 2 n = 1 n = 4 n = 5 n = 2 
 26,181 (23) 28,951 (77) 33,052 (31) 30,866 (136) 189,479 282,665 (25) 205,014 (41) 485,325 (70) 
Dose selection cohortDose escalation cohort
25 mg25 mg50 mg100 mg200 mg350 mg400 mg500 mg
PK parameter, n (%)n = 3n = 6n = 4n = 3n = 3n = 7n = 6n = 4
Cmax, ng/mLa n = 2 n = 5 n = 4 n = 3 n = 3 n = 4 n = 6 n = 2 
 1,459 (13) 1,770 (65) 2,336 (43) 2,882 (85) 13,175 (6) 16,452 (9) 15,078 (55) 29,530 (71) 
Tmax, hour, median (range) n = 2 n = 5 n = 4 n = 3 n = 3 n = 4 n = 6 n = 2 
 8.96 (8.03, 9.88) 4.05 (3.00, 6.07) 4.05 (4.03, 6.00) 3.25 (3.10, 8.02) 23.9 (3.22, 23.92) 3.60 (2.03, 8.85) 2.01 (1.02, 5.80) 6.33 (2.58, 10.08) 
AUC(0–τ), hour·ng/mLa n = 2 n = 4 n = 4 n = 2 n = 1 n = 4 n = 5 n = 2 
 26,181 (23) 28,951 (77) 33,052 (31) 30,866 (136) 189,479 282,665 (25) 205,014 (41) 485,325 (70) 

AUC(0–τ), area under the time concentration–time curve over the dosing interval; Cmax, maximum observed plasma concentration; CVb, coefficient of biological variation; PK, pharmacokinetic; Tmax, time to reach Cmax.

aData presented as geometric mean (CVb%).

GSK2636771 doses above 200 mg consistently resulted in blood concentrations greater than 0.6 μg/mL, the level predicted to robustly inhibit PI3Kβ from preclinical experiments. At the RP2D of 400 mg once daily, pre-dose concentrations remained above 3.04 μg/mL from Week 2 onward, Tmax ranged between 1.02 and 5.8 hours post-dose, and AUC(0–τ) had a geometric mean of 205 μg/mL.

Inhibition of PI3K signaling was observed in PRP with GSK2636771 doses of 100 to 500 mg once daily; the median percentage decrease from baseline at all post-dose time points was ≥61% for pSer473/Total AKT (Fig. 2A) and ≥60% for pSer9/Total GSK3β (Fig. 2B). At day 1, the inhibitory effects were shown to be greatest 1 to 2 hours post-dose with inhibition duration increasing from 10 to 24 hours at GSK2636771 doses ≥100 mg (data not shown).

Figure 2.

GSK2636771 inhibits PI3K signaling at doses of 100 to 500 mg once daily. Median values of pAKT/total AKT ratio (A) and pGSK3β/total GSK3β (B) were measured in platelet rich plasma on day 1, cycle 1, using Meso Scale Discovery electrochemiluminescent assay (n = 3 replicates per sample). C, Changes in pSer473, pThr246 PRAS40, pSer235/236, and pThr308 were measured in tumor biopsies using IHC (H-scores) at pretreatment and days 8 to 15 (2–4 hours post-dose; n = 1). D, Representative photomicrographs (20× magnification) of IHC staining showing tumor PD effects of p-AKT (Ser473, Thr308), p-PRAS40 (Thr246), and pSer6RP (Ser235/236) for two patients treated with GSK2636771 400 mg once daily (melanoma and TNBC). Biopsies were collected 2 to 4 hours post-dose between day 8 and 15. *, P < 0.05 Paired t-test. PD, pharmacodynamics; TNBC, triple negative brain cancer.

Figure 2.

GSK2636771 inhibits PI3K signaling at doses of 100 to 500 mg once daily. Median values of pAKT/total AKT ratio (A) and pGSK3β/total GSK3β (B) were measured in platelet rich plasma on day 1, cycle 1, using Meso Scale Discovery electrochemiluminescent assay (n = 3 replicates per sample). C, Changes in pSer473, pThr246 PRAS40, pSer235/236, and pThr308 were measured in tumor biopsies using IHC (H-scores) at pretreatment and days 8 to 15 (2–4 hours post-dose; n = 1). D, Representative photomicrographs (20× magnification) of IHC staining showing tumor PD effects of p-AKT (Ser473, Thr308), p-PRAS40 (Thr246), and pSer6RP (Ser235/236) for two patients treated with GSK2636771 400 mg once daily (melanoma and TNBC). Biopsies were collected 2 to 4 hours post-dose between day 8 and 15. *, P < 0.05 Paired t-test. PD, pharmacodynamics; TNBC, triple negative brain cancer.

Close modal

In four of five (80%) patients who received the RP2D of 400 mg once daily, decreases in pSer473 AKT and its downstream target (pThr246 PRAS40) were observed in paired tumor biopsies (pre- and on-treatment; Fig. 2C and D). Decreases in pSer235/236 S6RP and pThr308 AKT were also observed in two of four (50%) and two of five (40%) of evaluable patients, respectively (Fig. 2C and D).

Antitumor activity.

Of the total 65 enrolled patients, we observed one partial response (PR), 21 stable disease (SD); three noncomplete response (CR)/non-progressive disease (PD); 35 PD and five non-evaluable investigator-assessed best responses (based on RECIST 1.1 criteria). No CRs were reported. One patient with metastatic CRPC treated with GSK2636771 200 mg once daily during the dose-escalation phase experienced a confirmed PR, as well as a 78% fall in his prostate-specific antigen levels. The response was durable with progression after 68 weeks of treatment (Fig. 3A–C). An additional 10 patients treated with GSK2636771 remained on therapy and free of progression for at least 24 weeks, including two more patients with CRPC (34 and 57 weeks), five colorectal (25–33 weeks), one endometrial (33 weeks), one gastric (25 weeks), and one non–small cell lung cancer (33 weeks).

Figure 3.

Summary of antitumor activity of GSK2636771 in the first-in-man trial, and association with putative predictive biomarkers. A, Treatment duration in the 65 patients showed that one patient with CRPC had a PR, 24 patients had SD or non-CR/non-PD, seven of whom received GSK2636771 treatment for ≥6 months. B, The PR was observed at week 24 in the patient with CRPC, and was accompanied by a 78% reduction in PSA levels (C), which was durable with progression after 68 weeks of treatment (n = 1). D, Summary of data for 55 patients whose tumor tissue was analyzed retrospectively for somatic mutations and copy-number alterations: one copy loss (light green), two copy loss (dark green), gain (light red), amplification (dark red), coding mutations (M), noncoding mutations (M*), no data (gray). CNV was determined based on Nanostring or qPCR platforms. CNS, central nervous system; CNV, copy-number variation; CRPC, castrate-resistant prostate cancer; GE, gastroesophageal junction; PD, pharmacodynamics; PR, partial response; qPCR, quantitative PCR.

Figure 3.

Summary of antitumor activity of GSK2636771 in the first-in-man trial, and association with putative predictive biomarkers. A, Treatment duration in the 65 patients showed that one patient with CRPC had a PR, 24 patients had SD or non-CR/non-PD, seven of whom received GSK2636771 treatment for ≥6 months. B, The PR was observed at week 24 in the patient with CRPC, and was accompanied by a 78% reduction in PSA levels (C), which was durable with progression after 68 weeks of treatment (n = 1). D, Summary of data for 55 patients whose tumor tissue was analyzed retrospectively for somatic mutations and copy-number alterations: one copy loss (light green), two copy loss (dark green), gain (light red), amplification (dark red), coding mutations (M), noncoding mutations (M*), no data (gray). CNV was determined based on Nanostring or qPCR platforms. CNS, central nervous system; CNV, copy-number variation; CRPC, castrate-resistant prostate cancer; GE, gastroesophageal junction; PD, pharmacodynamics; PR, partial response; qPCR, quantitative PCR.

Close modal

Association between antitumor activity and genomic biomarkers

Archival or fresh tumor biopsy samples from 55 patients participating in the study were retrieved. Of those, 48 (87%) passed quality control for next-generation sequencing. Overall, five patients had PIK3CB copy-number gains or a putatively activating mutation (Fig. 3D and Supplementary Tables S7–S9); upon including the two additional patients with known PIK3CB aberrations at enrollment, three of seven (43%) patients with PIK3CB genomic aberrations were on trial for ≥6 months. All three of these patients had CRPC, and remained on GSK2636771 treatment for 34, 57, and 68 weeks, respectively (the latter being the single patient with a PIK3CB copy-number gain, exhibiting an investigator-assessed radiological PR). A patient with cervical cancer harboring a known PIK3CB copy-number gain received 22 weeks of GSK2636771 treatment, and showed a differential radiological response in lymph nodes (best overall response: SD).

Finally, we analyzed the frequency of genomic events in other tumor suppressor and cancer promoting genes in the trial population. PIK3CA activating mutations were identified in 11 patients, with one additional patient showing a PIK3CA copy gain; nine of these tumors harbored mutations in the RAS/RAF pathway, three in ataxia telangiectasia mutated (ATM), a key element of DNA damage response, and one in BRAF (Fig. 3D). Interestingly, all but one of these were mutually exclusive with PIK3CB aberrations in this population and did not correlate with antitumor responses.

The p.L1049R mutation identified in a patient with CRPC, who achieved prolonged SD, is homologous to the very well-characterized PIK3CA hotspot mutation p.H1047R reported in cancer (Supplementary Fig. S4A and S4B). To assess the functional relevance of the p.L1049R PIK3CB mutation, we transduced PC3 cells with pHTBBV1.1 (using baculovirus gene transfer into mammalian cells) expressing WT or the p.L1049R mutant at a range of multiplicity of infections. After 12 hours, substantially higher levels of pSer473 AKT were observed in cells with the p.L1049R mutation compared with WT cells (Supplementary Fig. S4C), suggesting an activating and potentially driving function for this mutation. Similar findings have now been reported for other mutations in the same region of PIK3CB (30, 31).

We report here on a first-time-in-human trial of GSK2636771, an oral selective PI3Kβ inhibitor. DLTs were identified and guided the selection of the RP2D, which was also supported by PK/PD data. Renal tubular damage, presenting in the form of hypophosphatemia, hypocalcemia, and proteinuria, was dose dependent, reversible, and manageable. Hyperglycemia and rash, typically reported for pan PI3K inhibitors, were uncommon. Also, GSK2636771 did not elevate insulin levels in mice compared with a pan PI3K/mTOR inhibitor. Furthermore, no hemorrhagic events or coagulation alterations were observed, despite preclinical data indicating that PI3Kβ plays an important role in adenosine diphosphate-induced platelet aggregation (32).

Target inhibition was demonstrated at tolerated doses. Repeat-dose exposure appeared to increase in a generally dose-proportional manner. GSK2636771 doses >200 mg once daily consistently resulted in blood concentrations >0.6 μg/mL, the level predicted to robustly inhibit PI3Kβ from preclinical experiments. The observed inhibitory effect of GSK2636771 on pAKT (Ser473) and other biochemical markers (e.g., pGSK3β [Ser9]) in PRP confirmed an effective modulation of the PI3K pathway across doses. The RP2D of 400 mg once daily was selected based on safety data. Significant target inhibition observed in tumor biopsies at this dose supported its selection.

Several genomic landscape studies of different tumor types have identified that the PI3K/AKT pathway is altered in squamous cell lung (33), endometrial and head and neck cancers (34), and advanced prostate (35) and ovarian (36) cancers. However, in tumor types where activation of PIK3CA is more common, such as breast or colorectal cancer, genomic aberrations in PIK3CB are rare (<2%) (3, 37). We, therefore, pursued retrospective tumor-targeted next-generation sequencing to explore putative predictive biomarkers of antitumor activity.

Activating mutations in PIK3CA have been previously associated to responses to pathway inhibitors (38); mutations leading to activation of PIK3CB have been reported in different tumor types, but their clinical relevance remains to date unknown (31, 39). Of 48 samples analyzed in this study, interestingly, five (10%) had PIK3CB aberrations, namely four patients with copy-number gains and one with an activating mutation (p.L1049R). In addition, two patients harboring PIK3CB copy-number gains that were previously determined were also enrolled in the expansion phase. Among these seven patients, we observed one durable radiological PR (on treatment for 68 weeks) and prolonged SD (on treatment for 34 and 57 weeks) in the CRPC subset. This association, albeit preliminary, is of particular interest in advanced prostate cancer, where molecular stratification for therapy selection remains an unmet medical need. However, the patient population investigated here was very small and further studies are needed to fully determine the role of PIK3CB aberrations and indeed other biomarkers in the molecular stratification of patients when targeting this pathway. Importantly, several patients without PIK3CB aberrations benefited from therapy with prolonged SD. Therefore, PIK3CB aberrations do not fully explain the responses observed, highlighting the complexity of the PI3K/AKT/mTOR signaling pathway, and the likely need for combination therapy to drive robust antitumor responses. The study was limited by the pre-selection of patients with PTEN-deficient tumors, which did not allow for assessment of the impact of genetic aberrations in non-PTEN–deficient tumors. Moreover, mostly archival rather than fresh tumor biopsies were analyzed which precluded the detection of aberrations that may emerge during tumor evolution (35, 40). Despite this, the preliminary results presented here are of interest and form the basis for further studies into the association of PIK3CB aberrations with clinical benefit. Further studies will also be needed to better characterize the mechanisms associated with tubular damage at high doses, although these were not a concern at the RP2D.

In conclusion, 400 mg once-daily continuous dosing was established as the RP2D for GSK2636771 based on DLTs. The safety profile of GSK2636771 400 mg once daily, together with proof-of-target modulation and the preliminary association of clinical benefit with PIK3CB genomic aberrations, support the continued evaluation of this compound in phase II clinical trials. The antitumor activity of GSK2636771 is being further studied as a single agent in molecularly defined populations within the NCI-MATCH clinical trial, in combination with the androgen receptor antagonist enzalutamide (Xtandi) in CRPC, in combination with paclitaxel in gastric cancer, and in combination with immunotherapy in melanoma.

D.A. Smith is an employee of and holds ownership interest (including patents) in GlaxoSmithKline. M. Motwani is an employee of GlaxoSmithKline. J.R. Infante is a consultant/advisory board member for ARMO, Bio Med Valley, and Novartis. J.S. de Bono is a consultant/advisory board member for GlaxoSmithKline. No potential conflicts of interest were disclosed by the other authors.

Conception and design: G. Ganji, H.A. Burris, R. Kumar, Y.-J. Bang, J.R. Infante, L. Yan, J.S. de Bono, H.-T. Arkenau

Development of methodology: C. Lemech, H.A. Burris, K. Swales, S. Decordova, J. Wu, J.R. Infante, L. Yan, J.S. de Bono, H.-T. Arkenau

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): J. Mateo, C. Lemech, H.A. Burris, S.-W. Han, K. Swales, S. Decordova, M. Phillip DeYoung, S. Kalyana-Sundaram, M. Motwani, J.M. Tolson, S.Y. Rha, H.C. Chung, J.P. Eder, S. Sharma, Y.-J. Bang, J.R. Infante, L. Yan, J.S. de Bono, H.-T. Arkenau

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): J. Mateo, C. Lemech, H.A. Burris, K. Swales, S. Decordova, M. Phillip DeYoung, D.A. Smith, S. Kalyana-Sundaram, J. Wu, M. Motwani, R. Kumar, J.M. Tolson, H.C. Chung, Y.-J. Bang, J.R. Infante, L. Yan, J.S. de Bono, H.-T. Arkenau

Writing, review, and/or revision of the manuscript: J. Mateo, G. Ganji, C. Lemech, H.A. Burris, S.-W. Han, K. Swales, S. Decordova, M. Phillip DeYoung, D.A. Smith, S. Kalyana-Sundaram, J. Wu, M. Motwani, R. Kumar, J.M. Tolson, S.Y. Rha, H.C. Chung, J.P. Eder, S. Sharma, Y.-J. Bang, J.R. Infante, L. Yan, J.S. de Bono, H.-T. Arkenau

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): C. Lemech, H.A. Burris, K. Swales, S. Decordova, M. Motwani, H.C. Chung, J.S. de Bono

Study supervision: C. Lemech, J.M. Tolson, H.C. Chung, S. Sharma, J.R. Infante, L. Yan, J.S. de Bono, H.-T. Arkenau

This study was funded by GSK. Their role included study concept and design, funding of participating centres, analysis of data, development, and funding of the final report and manuscript. The authors acknowledge the contributions of Wendy Halsey and Harjeet van der Keyl (genomics), Ralph Rivero and his team (chemistry), and Jen Ariazi, Chris Sherk, and Ramona Plant (preclinical biology). They would also like to acknowledge Min-kyu Jung, Hyu Song Kim, Roy Herbst, and Todd Bauer for site participation and patient recruitment. Medical writing and editorial assistance were provided by Paul O'Regan, PhD, Katy Tucker, PhD, CMPP, and Clare Slater, PhD, CMPP of Fishawack Indicia Ltd., UK, also funded by GSK.

We acknowledge support from Cancer Research UK to The Institute of Cancer Research and the Royal Marsden Cancer Centre (grant No. C51/A6883); from an Experimental Cancer Medicine Centres network grant; and from a National Institute for Health Research Biomedical Research Centre grant to The Royal Marsden. J. Mateo was supported by a Prostate Cancer Foundation Young Investigator Award and a Medical Research Council/Prostate Cancer UK-Movember Foundation fellowship. K. Swales is supported by Cancer Research UK (grant No. C347/A18077). This study was funded by GSK.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1.
Liu
P
,
Cheng
H
,
Roberts
TM
,
Zhao
JJ
. 
Targeting the phosphoinositide 3-kinase pathway in cancer
.
Nat Rev Drug Discov
2009
;
8
:
627
44
.
2.
Koboldt
DC
,
Fulton
RS
,
McLellan
MD
,
Schmidt
H
,
Kalicki-Veizer
J
,
McMichael
JF
, et al
Comprehensive molecular portraits of human breast tumours
.
Nature
2012
;
490
:
61
70
.
3.
Muzny
DM
,
Bainbridge
MN
,
Chang
K
,
Dinh
HH
,
Drummond
JA
,
Fowler
G
, et al
Comprehensive molecular characterization of human colon and rectal cancer
.
Nature
2012
;
487
:
330
7
.
4.
Di Cristofano
A
,
Pandolfi
PP
. 
The multiple roles of PTEN in tumor suppression
.
Cell
2000
;
100
:
387
90
.
5.
Samuels
Y
,
Wang
Z
,
Bardelli
A
,
Silliman
N
,
Ptak
J
,
Szabo
S
, et al
High frequency of mutations of the PIK3CA gene in human cancers
.
Science
2004
;
304
:
554
.
6.
Courtney
KD
,
Corcoran
RB
,
Engelman
JA
. 
The PI3K pathway as drug target in human cancer
.
J Clin Oncol
2010
;
28
:
1075
83
.
7.
Rodon
J
,
Dienstmann
R
,
Serra
V
,
Tabernero
J
. 
Development of PI3K inhibitors: lessons learned from early clinical trials
.
Nat Rev Clin Oncol
2013
;
10
:
143
53
.
8.
Chandarlapaty
S
,
Sawai
A
,
Scaltriti
M
,
Rodrik-Outmezguine
V
,
Grbovic-Huezo
O
,
Serra
V
, et al
AKT inhibition relieves feedback suppression of receptor tyrosine kinase expression and activity
.
Cancer Cell
2011
;
19
:
58
71
.
9.
Stratikopoulos
EE
,
Dendy
M
,
Szabolcs
M
,
Khaykin
AJ
,
Lefebvre
C
,
Zhou
MM
, et al
Kinase and BET inhibitors together clamp inhibition of PI3K signaling and overcome resistance to therapy
.
Cancer Cell
2015
;
27
:
837
51
.
10.
Ihle
NT
,
Lemos
R
 Jr
,
Wipf
P
,
Yacoub
A
,
Mitchell
C
,
Siwak
D
, et al
Mutations in the phosphatidylinositol-3-kinase pathway predict for antitumor activity of the inhibitor PX-866 whereas oncogenic Ras is a dominant predictor for resistance
.
Cancer Res
2009
;
69
:
143
50
.
11.
Busaidy
NL
,
Farooki
A
,
Dowlati
A
,
Perentesis
JP
,
Dancey
JE
,
Doyle
LA
, et al
Management of metabolic effects associated with anticancer agents targeting the PI3K-Akt-mTOR pathway
.
J Clin Oncol
2012
;
30
:
2919
28
.
12.
Engelman
JA
,
Luo
J
,
Cantley
LC
. 
The evolution of phosphatidylinositol 3-kinases as regulators of growth and metabolism
.
Nat Rev Genet
2006
;
7
:
606
19
.
13.
Brachmann
SM
,
Kleylein-Sohn
J
,
Gaulis
S
,
Kauffmann
A
,
Blommers
MJ
,
Kazic-Legueux
M
, et al
Characterization of the mechanism of action of the pan class I PI3K inhibitor NVP-BKM120 across a broad range of concentrations
.
Mol Cancer Ther
2012
;
11
:
1747
57
.
14.
Jia
S
,
Liu
Z
,
Zhang
S
,
Liu
P
,
Zhang
L
,
Lee
SH
, et al
Essential roles of PI(3)K-p110beta in cell growth, metabolism and tumorigenesis
.
Nature
2008
;
454
:
776
9
.
15.
Ni
J
,
Liu
Q
,
Xie
S
,
Carlson
C
,
Von
T
,
Vogel
K
, et al
Functional characterization of an isoform-selective inhibitor of PI3K-p110beta as a potential anticancer agent
.
Cancer Discov
2012
;
2
:
425
33
.
16.
Schwartz
S
,
Wongvipat
J
,
Trigwell
CB
,
Hancox
U
,
Carver
BS
,
Rodrik-Outmezguine
V
, et al
Feedback suppression of PI3Kalpha signaling in PTEN-mutated tumors is relieved by selective inhibition of PI3Kbeta
.
Cancer Cell
2015
;
27
:
109
22
.
17.
Wee
S
,
Wiederschain
D
,
Maira
SM
,
Loo
A
,
Miller
C
,
deBeaumont
R
, et al
PTEN-deficient cancers depend on PIK3CB
.
Proc Natl Acad Sci U S A
2008
;
105
:
13057
62
.
18.
Foukas
LC
,
Claret
M
,
Pearce
W
,
Okkenhaug
K
,
Meek
S
,
Peskett
E
, et al
Critical role for the p110alpha phosphoinositide-3-OH kinase in growth and metabolic regulation
.
Nature
2006
;
441
:
366
70
.
19.
Carver
BS
,
Chapinski
C
,
Wongvipat
J
,
Hieronymus
H
,
Chen
Y
,
Chandarlapaty
S
, et al
Reciprocal feedback regulation of PI3K and androgen receptor signaling in PTEN-deficient prostate cancer
.
Cancer Cell
2011
;
19
:
575
86
.
20.
Ferraldeschi
R
,
Nava Rodrigues
D
,
Riisnaes
R
,
Miranda
S
,
Figueiredo
I
,
Rescigno
P
, et al
PTEN protein loss and clinical outcome from castration-resistant prostate cancer treated with abiraterone acetate
.
Eur Urol
2015
;
67
:
795
802
.
21.
Reid
AH
,
Attard
G
,
Danila
DC
,
Oommen
NB
,
Olmos
D
,
Fong
PC
, et al
Significant and sustained antitumor activity in post-docetaxel, castration-resistant prostate cancer with the CYP17 inhibitor abiraterone acetate
.
J Clin Oncol
2010
;
28
:
1489
95
.
22.
Bosch
A
,
Li
Z
,
Bergamaschi
A
,
Ellis
H
,
Toska
E
,
Prat
A
, et al
PI3K inhibition results in enhanced estrogen receptor function and dependence in hormone receptor-positive breast cancer
.
Sci Transl Med
2015
;
7
:
283ra51
.
23.
Crowder
RJ
,
Phommaly
C
,
Tao
Y
,
Hoog
J
,
Luo
J
,
Perou
CM
, et al
PIK3CA and PIK3CB inhibition produce synthetic lethality when combined with estrogen deprivation in estrogen receptor-positive breast cancer
.
Cancer Res
2009
;
69
:
3955
62
.
24.
Serra
V
,
Scaltriti
M
,
Prudkin
L
,
Eichhorn
PJ
,
Ibrahim
YH
,
Chandarlapaty
S
, et al
PI3K inhibition results in enhanced HER signaling and acquired ERK dependency in HER2-overexpressing breast cancer
.
Oncogene
2011
;
30
:
2547
57
.
25.
Zhang
Y
,
Moerkens
M
,
Ramaiahgari
S
,
de Bont
H
,
Price
L
,
Meerman
J
, et al
Elevated insulin-like growth factor 1 receptor signaling induces antiestrogen resistance through the MAPK/ERK and PI3K/Akt signaling routes
.
Breast Cancer Res
2011
;
13
:
R52
.
26.
Bantscheff
M
,
Eberhard
D
,
Abraham
Y
,
Bastuck
S
,
Boesche
M
,
Hobson
S
, et al
Quantitative chemical proteomics reveals mechanisms of action of clinical ABL kinase inhibitors
.
Nat Biotechnol
2007
;
25
:
1035
44
.
27.
Scher
HI
,
Halabi
S
,
Tannock
I
,
Morris
M
,
Sternberg
CN
,
Carducci
MA
, et al
Design and end points of clinical trials for patients with progressive prostate cancer and castrate levels of testosterone: recommendations of the Prostate Cancer Clinical Trials Working Group
.
J Clin Oncol
2008
;
26
:
1148
59
.
28.
Eisenhauer
EA
,
Therasse
P
,
Bogaerts
J
,
Schwartz
LH
,
Sargent
D
,
Ford
R
, et al
New response evaluation criteria in solid tumours: revised RECIST guideline (version 1.1)
.
Eur J Cancer
2009
;
45
:
228
47
.
29.
Dienstmann
R
,
Rodon
J
,
Serra
V
,
Tabernero
J
. 
Picking the point of inhibition: a comparative review of PI3K/AKT/mTOR pathway inhibitors
.
Mol Cancer Ther
2014
;
13
:
1021
31
.
30.
Nakanishi
Y
,
Walter
K
,
Spoerke
JM
,
O'Brien
C
,
Huw
LY
,
Hampton
GM
, et al
Activating mutations in PIK3CB confer resistance to PI3K inhibition and define a novel oncogenic role for p110beta
.
Cancer Res
2016
;
76
:
1193
203
.
31.
Pazarentzos
E
,
Giannikopoulos
P
,
Hrustanovic
G
,
St John
J
,
Olivas
VR
,
Gubens
MA
, et al
Oncogenic activation of the PI3-kinase p110beta isoform via the tumor-derived PIK3Cbeta(D1067V) kinase domain mutation
.
Oncogene
2016
;
35
:
1198
205
.
32.
Garcia
A
,
Kim
S
,
Bhavaraju
K
,
Schoenwaelder
SM
,
Kunapuli
SP
. 
Role of phosphoinositide 3-kinase beta in platelet aggregation and thromboxane A2 generation mediated by Gi signalling pathways
.
Biochem J
2010
;
429
:
369
77
.
33.
Hammerman
PS
,
Lawrence
MS
,
Voet
D
,
Jing
R
,
Cibulskis
K
,
Sivachenko
A
, et al
Comprehensive genomic characterization of squamous cell lung cancers
.
Nature
2012
;
489
:
519
25
.
34.
Lawrence
MS
,
Sougnez
C
,
Lichtenstein
L
,
Cibulskis
K
,
Lander
E
,
Gabriel
SB
, et al
Comprehensive genomic characterization of head and neck squamous cell carcinomas
.
Nature
2015
;
517
:
576
82
.
35.
Robinson
D
,
Van Allen
EM
,
Wu
YM
,
Schultz
N
,
Lonigro
RJ
,
Mosquera
JM
, et al
Integrative clinical genomics of advanced prostate cancer
.
Cell
2015
;
161
:
1215
28
.
36.
Bell
D
,
Berchuck
A
,
Birrer
M
,
Chien
J
,
Cramer
DW
,
Dao
F
, et al
Integrated genomic analyses of ovarian carcinoma
.
Nature
2011
;
474
:
609
15
.
37.
Ciriello
G
,
Gatza
ML
,
Beck
AH
,
Wilkerson
MD
,
Rhie
SK
,
Pastore
A
, et al
Comprehensive molecular portraits of invasive lobular breast cancer
.
Cell
2015
;
163
:
506
19
.
38.
Janku
F
,
Wheler
JJ
,
Naing
A
,
Falchook
GS
,
Hong
DS
,
Stepanek
VM
, et al
PIK3CA mutation H1047R is associated with response to PI3K/AKT/mTOR signaling pathway inhibitors in early-phase clinical trials
.
Cancer Res
2013
;
73
:
276
84
.
39.
Dbouk
HA
,
Khalil
BD
,
Wu
H
,
Shymanets
A
,
Nurnberg
B
,
Backer
JM
. 
Characterization of a tumor-associated activating mutation of the p110beta PI 3-kinase
.
PLoS One
2013
;
8
:
e63833
.
40.
Cancer Genome Atlas Research N
. 
The molecular taxonomy of primary prostate cancer
.
Cell
2015
;
163
:
1011
25
.