Dasatinib as a Single Agent in Triple-Negative Breast Cancer: Results of an Open-Label Phase 2 Study

Breast cancer is the most commonly diagnosed female cancer in the United States with approximately 207,000 new cases per year (3). Targeted therapies, including antiestrogens, aromatase inhibitors, and trastuzumab, have been effective against cancers expressing relevant target molecules, that is, estrogen receptor (ER), progesterone receptor (PgR), or HER2 (4, 5).

Triple-negative breast cancer (TNBC) is particularly challenging because tumors lack validated molecular targets. Molecular profiling has identified TNBC as a heterogeneous group including the basal-like breast cancer subtype (6, 7). Generally, TNBC is highly proliferative (8), has a poor outcome (7), is more likely to recur (9), and progresses rapidly (10). Treatment with cytotoxic drugs for TNBC is common, although durable disease control is rare (11). Therefore, identification of molecular targets for TNBC therapy is critical.

SRC is a ubiquitously expressed intracellular tyrosine kinase that regulates protein–protein interactions and signaling pathways involved in adhesion, migration, invasion, protection against apoptosis, and vascular endothelial growth factor expression (12, 13). Abnormal activation or amplification of SRC and SRC-family kinases (SFK) has been detected in various tumors, including breast cancer (12–14), and has been shown to play a role in proliferation, migration, and invasion of breast cancer cell lines (15, 16). SRC also regulates osteoclast function in healthy bone and bone metastases (17, 18). SRC therefore represents a rational molecular target for breast cancer treatment, including TNBC.

Dasatinib, a potent oral inhibitor of SFKs, is approved for treating patients with Philadelphia chromosome-positive leukemias (19). Preclinical studies showed growth inhibition of breast cancer cells with a basal-like phenotype by single-agent dasatinib more frequently than other phenotypes, also identifying specific markers of response (15, 20). Preclinical TNBC models also showed synergistic or additive dasatinib activity with chemotherapy (21), suggesting that dasatinib may provide clinical benefit in TNBC.

We report results from CA180059, a prospective, open label, phase 2 trial of twice-daily (BID) dasatinib in patients with locally advanced or metastatic TNBC that had progressed after prior chemotherapy for advanced disease.

Eligible patients were female with an Eastern Cooperative Oncology Group performance status of 0 to 1, and had measurable recurrent or progressive locally advanced or metastatic TNBC that had progressed after anthracycline and/or taxane therapy (maximum of 2 prior chemotherapy regimens for advanced disease). Tumor receptor status was determined using immunohistochemistry (IHC) analysis from a previous biopsy showing ER/PgR negativity (<10% of cells with positive staining) and HER2-normal staining (IHC 0–1+ or negative by chromogenic/fluorescent in situ hybridization). At the time of study initiation, the definition of ER/PgR positivity was driven by response to hormone therapy and thus the cutoff was less than 10% ER/PgR by IHC. The current American Society of Clinical Oncology-College of American Pathologists (ASCO-CAP) guidelines are more stringent and may better define this group of patients (22). Women of childbearing potential were required to have adequate contraception during study. Patients were excluded if they were pregnant or breast feeding, had symptomatic central nervous system metastases, or bone metastases only, had any antineoplastic therapy within 14 days prior to starting dasatinib, had reduced hematologic, hepatic, or renal function, pleural effusion or any concurrent medical condition potentially increasing toxicity risk. Eligible patients had adequate liver function [grade 0–1 aspartate aminotransferase (AST), alanine aminotransferase (ALT), and total bilirubin], serum Ca²⁺ of lower limit of normal or more, grade 0 to 1 serum K⁺, Mg²⁺, and phosphate, grade 0 to 2 creatinine, and grade 0 to 1 hemoglobin, neutrophil count, platelet count, prothrombin time, and partial thromboplastin time. Patients were excluded if they had any concurrent medical condition which may have increased toxicity risk, including pleural or pericardial effusion, clinically significant coagulation or platelet function disorder, infection requiring intravenous antibiotics, requirement for prohibited concomitant therapy, ongoing or recent (≤3 months) significant gastrointestinal bleeding, or clinically significant cardiovascular disease.

Trials were conducted in accordance with the Declaration of Helsinki and approved by responsible Institutional Review Boards or Ethics Committees of participating centers. All patients gave written informed consent.

The primary objective of this open label phase 2 study was to estimate objective response rate (ORR). Secondary objectives were to estimate response duration, disease control rate (DCR) at week 17, overall progression-free survival (PFS), safety and tolerability, and pharmacokinetics. A Gehan 2-stage design, involving 29 and 16 response evaluable patients in the first and second stages, respectively, was used. If no response was observed in the first stage, the study would have been closed to accrual with the conclusion that an ORR of 10% or more was unlikely (95% CI). Initially, dasatinib was administered as 100 mg BID, which was decreased to 70 mg BID by protocol amendment after the first 23 patients.

Dasatinib dose was interrupted, reduced, or treatment suspended in the event of drug-related grade 3 to 4 toxicity or recurrent unacceptable or unmanageable drug-related grade 2 toxicity. Dasatinib was reduced to either 50 mg BID, or 100 mg once daily (QD) based on individual tolerability. Patients were discontinued on recurrence of unacceptable toxicity with 50 mg BID dosing, confirmed progressive disease (PD), or excessive toxicity despite dose reduction. Dose reescalation or reinitiation was permitted at investigator discretion once toxicity had decreased to grade 0 to 1.

Patients were evaluated at least every 2 weeks ± 4 days for the first 8 weeks (weeks 3, 5, 7, and 9), every 4 weeks for 2 visits (after 3 and 4 months on treatment), and every 8 weeks ± 1 week until the end of treatment. Tumor responses were classified according to Response Evaluation Criteria in Solid Tumor (RECIST) guidelines (23) defined as: complete response (CR) = disappearance of all target and nontarget lesions confirmed after 4 weeks or more; partial response (PR) = 30% or more decrease from baseline in the sum of largest diameters (SLD) of target lesions confirmed after 4 weeks or more; PD = appearance of new lesions, 20% or more increase in the SLD of target lesions, or unequivocal progression of nontarget lesions; and stable disease = insufficient change to qualify as PR or PD without unequivocal progression of nontarget lesions after 6 weeks or more on-study. Patients with equivocal evidence of PD remained on-study at investigator's discretion. ORR was defined as the proportion of response evaluable patients with CR or PR as best response. DCR was defined as the proportion of response evaluable patients with CR or PR as best response or with stable disease for 16 weeks or more. The response evaluable population included nonresponding patients who discontinued therapy for toxicity, even if no on-study assessment was carried out. Duration of response was measured from the first date that CR or PR criteria were met until PD. PFS, assessed in all treated patients, was defined as time from first dose until PD. Patients who died without documented PD were considered to have progressed at date of death; those who neither progressed nor died were censored at last tumor evaluation. Kaplan–Meier analysis was used to calculate median values and 95% CI for response duration and PFS.

Adverse events (AE) were graded according to National Cancer Institute Common Terminology Criteria for Adverse Events version 3.0, with causal relationship assigned by investigators.

Because pharmacokinetic information for dasatinib 70 mg BID is already available, pharmacokinetic assessment was optional (24, 25). Blood samples were collected at 0, 1, 3, 6, and 12 hours at week 3 and week 7 or week 9. Mean plasma dasatinib concentrations and SDs were assayed by tandem liquid chromatography mass spectrometry.

Forty-four of 55 enrolled patients were treated. Of the 11 patients who were not treated, 10 were found to be ineligible and one withdrew consent prior to treatment. Of the 44 treated patients, 43 were response evaluable (1 patient was withdrawn without progression for grade 1 pneumonia unrelated to treatment). Four response evaluable patients discontinued treatment before on-study tumor assessment because of study drug-related toxicity. Patient demographics are reported in Table 1. Documentation of confirmed PR (cPR) in the 16th and 25th treated patients satisfied protocol-defined criteria for continued accrual. All patients discontinued treatment because of disease progression (n = 26, 59%), study drug toxicity (n = 8, 18%), or other reasons (n = 10, 23%; Table 1).

Initially, 23 patients received dasatinib at 100 mg BID; following protocol amendment, 21 patients received 70 mg BID. In the 100 mg BID group, median daily dose was 146 mg/d (range 87–200), and 39% of patients received 85% or more of their planned dose. In the 70 mg BID group, median daily dose was 135 mg/d (range 87–140) and 86% of patients received 85% or more of their planned dose. Median therapy duration in both groups was 1.61 months excluding interruptions, and median durations were 1.35 and 1.61 months in the 100 and 70 mg BID groups, respectively. Only 3 patients continued dasatinib beyond week 17.

A higher percentage of patients received dose interruptions in the 100 mg BID group (n = 19; 83%) than in the 70 mg BID group (n = 11; 52%), and 2 or more interruptions were required by 43% (n = 10) and 19% (n = 4), respectively. In the 100 mg BID group, 18 first dose interruptions were due to nonhematologic toxicity; those occurring in more than 2 patients were pleural effusion (n = 3) and diarrhea (n = 4). In the 70 mg BID group, all first dose interruptions (n = 11) were due to nonhematologic toxicity, those occurring in more than 1 patient were chest-wall pain (n = 2) and vomiting (n = 2).

Dose reduction was carried out in 14 patients receiving 100 mg BID (61%), but only 5 patients receiving 70 mg BID (24%); 2 or more dose reductions were carried out in 30% and 5%, respectively. In the 100 mg BID group, 9 first dose reductions were related to nonhematologic toxicity; those occurring in more than 1 patient were cough (n = 2) and asthenia (n = 2). In the 70 mg BID group, 2 first dose reductions were related to nonhematologic toxicity (diarrhea, pleural effusion, and headache). Median times to toxicity related first dose reduction or interruption were 18 and 14 days in the 100 and 70 mg BID groups, respectively.

Of 43 response evaluable patients, 2 cPRs were observed (response durations of 14 and 58 weeks), resulting in an ORR of 4.7% (2 of 43; 95% CI: 0.57–15.81). Additional details of the 2 patients who had PRs are as follows. Patient 19 had prior adjuvant chemotherapy, prior hormonal therapy for a previous contralateral ER⁺ tumor, and recurrent TNBC that was considered to have metastasized. She started dasatinib at 100 mg BID, reduced to 70 mg BID in week 5 due to grade 3 diarrhea, and reduced to 50 mg BID in week 24 due to grade 2 neutropenia. Normalization of tumor markers was reported as indicated by serum markers, resolution of bone pain, and regression of clinically palpable and radiographically measurable lymph nodes. However, bone scans were persistently positive, therefore the response was considered PR, first recorded in week 8 and lasting for 58 weeks. Patient 30 had prior treatment with chemotherapy and hormonal therapy, including gemcitabine plus capecitabine in the advanced setting, and recurrent disease in multiple lymph nodes. She started dasatinib at 100 mg BID, reduced to 70 mg BID in week 8, and to 50 mg BID in week 13 due to asthenia, then withdrew from the trial at week 17. Complete regression of a clinically measured lymph node and PR of a radiographically measured lymph node (>50% reduction in SLD) were recorded. PR was first recorded in week 8 and disease progression at week 22.

DCR was 4 of 43 (9.3%; 95% CI: 2.59–22.14), including 2 patients who had stable disease lasting 16 weeks or more (25 and 33 weeks). Twelve patients had stable disease as best response (Table 2). Tumor size decreased from baseline in 11 patients (Fig. 1A); the 2 patients who had a cPR had the greatest decreases in tumor size of −86% and −100% (the latter did not qualify as CR because of residual nontarget lesions). Median PFS for treated patients (n = 44) was 8.3 weeks (95% CI: 7.3–15.3). Proportions of treated patients with PFS at weeks 9, 17, and 25 were 0.37, 0.22, and 0.09, respectively (Fig. 1B).

Of 23 patients who received dasatinib 100 mg BID, 5 (22%) experienced a drug-related serious AE (SAE), 12 (52%) had a grade 3 drug-related AE, 19 (83%) required at least 1 dose interruption, and 14 (61%) had their dose reduced. Following protocol amendment to reduce the starting dose to 70 mg BID, dasatinib was reasonably well tolerated (Table 3) with only 5 patients (24%) requiring dose reduction.

The most common drug-related AEs (>25% incidence) were fatigue, nausea, dyspnea, diarrhea, pleural effusion, rash, vomiting, anorexia, cough, and headache. No grade 4 or 5 drug-related events were reported. Seven deaths due to disease progression were reported; the study did not require follow-up for survival. Fourteen patients (32%) reported SAEs of any grade, regardless of relationship to drug (11 in the 100 mg BID group and 3 in the 70 mg BID group). Drug-related SAEs of any grade were seen in 6 patients (14%), of which one occurred in the 70 mg BID group and 5 occurred in the 100 mg BID group, with only pericardial and pleural effusion occurring in more than 1 patient (Table 4).

On-study grade 3 to 4 laboratory abnormalities were uncommon. Except for grade 3 neutropenia in 3 patients, all hematologic abnormalities were grade 1 to 2. In the 70 mg BID group, 1 patient had on-study worsening of ALT and AST from grade 0 to 3. In the 100 mg BID group, 1 patient had on-study worsening in hypokalemia from baseline grade 0 to 4. Hypophosphatemia was seen in both groups (70 mg BID: grade 3, n = 2; 100 mg BID: grade 4, n = 1).

Of 10 patients who discontinued for AEs, 4 were in the 100 mg BID group and 6 were in the 70 mg BID group. Of 8 patients who discontinued for drug-related AEs (18%), 3 were in the 100 mg BID group (fatigue, headache, and myopericarditis) and 5 were in the 70 mg BID group (nausea, heartburn, and rash; nausea, fatigue, chest wall pain, insomnia, and anxiety; dyspnea; pleural effusion, and pneumonia; elevated ALT/AST).

Samples for pharmacokinetic analysis were collected from 22 patients (Table 5). Plasma concentrations of dasatinib predose and after 1, 3, and 6 hours in patients with TNBC were within the range of values obtained using the same dose regimens in patients with solid tumors (25) or leukemia (24).

Preclinical data suggest that dasatinib may be effective in treating TNBC by inhibiting proliferation, migration, and invasion of metastatic breast cancer. This article reports one of the first phase 2 studies of an SRC inhibitor in breast cancer and development of a targeted therapy in patients with TNBC, an entity whose definition is still being refined. This study uses a definition of TNBC that is not as stringent as the current ASCO-CAP guidelines would dictate.

Single-agent dasatinib showed limited efficacy in patients with TNBC. Overall, 2 of 43 patients (5%) achieved a RECIST PR, both occurring in lymph nodes and lasting for 14 and 58 weeks. In addition, 11 of 43 patients (26%) experienced stable disease, including 2 lasting for 25 and 33 weeks, and tumor size decreased from baseline in 11 of 36 evaluable patients (31%). Median PFS was 8.3 weeks and only 3 patients continued dasatinib beyond the week 17 assessment. Despite low response rates, a subpopulation of patients with TNBC may exist, whose tumors have increased sensitivity to SRC inhibition and could potentially achieve improved responses to dasatinib. It is increasingly apparent that TNBC is not a single disease but a heterogeneous group of tumors. Interpretation of this study is limited by an absence of biomarker data which could be used to determine which tumors are truly SRC dependent, and further biomarker development is required. To address the importance of identifying appropriate predictive markers, biomarker data from this study are being aggregated with data from other dasatinib studies.

SRC biology is complex and optimal use of SRC inhibitors in breast cancer remains to be determined. A recent study found that subcellular SRC localization and the site of SRC phosphorylation were associated with different breast cancer survival outcomes. Tumors with high cytoplasmic SRC levels or membrane-associated tyrosine-419 phosphorylated SRC were associated with decreased survival, whereas those with activated nuclear and cytoplasmic tyrosine-215 phosphorylated SRC had a significantly improved survival benefit (26). Preclinical evidence suggests that, as a result of complex crosstalk between SRC-regulated signaling pathways, SRC inhibition may result in a paradoxical increase in proliferative signaling in breast cancer cells (27). A 200-gene signature for breast cancer epithelial-mesenchymal transition has been identified, including the SFK member LYN, which is associated with shorter overall survival and the basal (triple negative) subtype. LYN inhibition by dasatinib inhibited TNBC cell invasion in vitro without inhibiting proliferation (16). Future studies will correlate treatment responses to dasatinib with investigational biomarkers in patients with breast cancer.

The limited efficacy observed in this trial is similar to those of other single-agent tyrosine kinase inhibitors in unselected patients with advanced breast cancer. For example, in two phase 2 studies of patients with metastatic breast cancer previously exposed to anthracyclines or taxanes treated with sorafenib, no patient had a PR or CR (28), and only 3 of 20 (15%) patients with TNBC treated with sunitinib had a PR (29). These results suggest that better patient selection strategies are required.

The dosing regimen for this study was based on a phase 1 study in which patients with solid tumors received dasatinib at a putative maximum-tolerated dose of 120 mg BID (25). However, almost all patients who received this dose in this study required treatment interruption. Therefore, the starting dose was reduced first to 100 mg BID, then to 70 mg BID, equal to the first dose reduction level for patients who could not tolerate 100 mg BID. Although this study was not designed to compare dosing groups, data suggest that dasatinib 70 mg BID had improved tolerability relative to dasatinib 100 mg BID, including numerically lower rates of treatment interruption, dose reduction, and grade 3 AEs. No grade 4 or 5 AEs were recorded in either dosing group. Importantly, AEs observed were in line with previous clinical experience with dasatinib and no unexpected AEs were reported. Median dasatinib dose intensities were similar in the 100 mg BID and 70 mg BID groups (146 and 135 mg/d, respectively). Overall median duration of study therapy was unexpectedly short (1.61 months), possibly because of discontinuation for toxicity.

To further evaluate the potential of dasatinib in breast cancer, several phase 2 trials are ongoing. These include biomarker-driven studies in TNBC (NCT00780676, NCT00546104, and NCT00817531) and studies evaluating dasatinib combined with chemotherapy in the metastatic setting (NCT00924352 and NCT00820170). For hormone receptor–positive breast cancer, dasatinib combined with endocrine therapy is being evaluated in patients who have received either no prior hormonal therapy in the metastatic setting (NCT00903006 and NCT00696072) or who have progressed following prior aromatase inhibitor therapy (NCT00767520 and NCT00754325). On the basis of data indicating that dasatinib inhibits osteoclast activity (30, 31), dasatinib effects on bone metastases will also be evaluated. Building on dose optimization trials in leukemia showing significantly improved tolerability and equivalent efficacy (32), dasatinib 100 mg QD is being administered in recently initiated studies. Future data will therefore provide a broader evaluation of the therapeutic potential of dasatinib-mediated SRC inhibition in patients with breast cancer.

R.S. Finn has received research grants from Bristol-Myers Squibb and Novartis, received honoraria from Genentech, and acted as a consultant or advisor for Bristol-Myers Squibb. C. Bengala, N. Ibrahim, and H. Roché have no conflicts of interest to disclose. J. Sparano has received honoraria from Bristol-Myers Squibb. L.J. Goldstein has acted as a consultant or advisor for Bristol-Myers Squibb. L.C. Strauss, J. Fairchild, and O. Sy are employees of and hold stock interests in Bristol-Myers Squibb.

We thank the patients and their families who participated in this trial and acknowledge the efforts of study staff at the investigational sites. Professional medical writing assistance was provided by StemScientific, funded by Bristol-Myers Squibb.

This study was sponsored by Bristol-Myers Squibb Oncology, Princeton, NJ. R.S. Finn is a recipient of an NIH-LRP Award.

This study was designed by employees of Bristol-Myers Squibb and R.S. Finn. Patient data were collected at participating institutes, analyzed using the sponsor‚s data management systems, and interpreted by the sponsor‚s statistical team in collaboration with the authors. Study data were available to all authors. An initial outline of the article was developed following discussions with the first and last authors by an independent company with funding provided by Bristol-Myers Squibb. All authors contributed to writing and reviewing of the manuscript and agreed with the decision to submit for publication.

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.