A Phase I Study of DLYE5953A, an Anti-LY6E Antibody Covalently Linked to Monomethyl Auristatin E, in Patients with Refractory Solid Tumors

The lymphocyte antigen 6 complex locus E gene (LY6E) encodes an IFN-inducible, glycophosphatidylinositol-anchored, surface-membrane protein known as LY6E, that has been identified as a promising target for antibody–drug conjugates (ADCs; ref. 1). Characterization of the function of LY6E remains incomplete, with potential roles in immune regulation, viral infection, and oncogenesis (2–5). Although there is limited LY6E expression in most normal tissues, frequently there is high LY6E expression (IHC 2/3+) in common epithelial cancers, including breast cancer (64%), pancreatic cancer (63%), non–small cell lung cancer (NSCLC, 56%), ovarian cancer (53%), head and neck squamous cell carcinoma (42%), and gastric cancer (31%), thus providing a strong rationale to develop an LY6E-targeting ADC for the treatment of LY6E-expressing solid tumors (1).

DLYE5953A is an ADC that comprises a humanized anti-LY6E monoclonal antibody (IgG₁: MLYE4489A) linked through a protease labile linker (maleimidocaproyl-valine-citrulline p-aminobenzyloxycarbonyl; ref. 6) to a potent antimitotic agent (monomethyl auristatin E, or MMAE; ref. 7). Upon binding to LY6E on the cell surface, DLYE5953A is internalized and delivered to lysosomes, where MMAE or MMAE-containing catabolites induce cell death (1). In this manner, ADCs have the potential to increase drug exposure, particularly to cancer cells exhibiting relatively high target expression, while minimizing exposure to normal tissue. In preclinical studies, DLYE5953A showed potent and selective inhibition of cell proliferation in LY6E-expressing cancer cell lines and mice xenograft models (1). Based on acceptable nonclinical pharmacokinetics (PK) and safety profiles, this first-in-human study was designed to characterize the safety, tolerability, and PK parameters of DLYE5953A in patients with solid tumors refractory to standard treatments. Preliminary assessments of DLYE5953A antitumor activity, its association with tumor LY6E expression, and mechanisms of acquired resistance were also performed.

This was an open-label, multicenter, phase I study using a 3 + 3 dose-escalation design to evaluate the safety, tolerability, recommended phase II dose (RP2D), PK, antidrug antibody (ADA) formation, preliminary antitumor activity, and potential biomarkers for DLYE5953A. Patients with advanced or metastatic solid tumors received DLYE5953A (supplied by Genentech, Inc.) intravenously at doses of 0.2, 0.4, 0.8, 1.6, or 2.4 mg/kg once every 3 weeks (Q3W). Dose-expansion cohorts were enrolled at the RP2D to further characterize the safety profile and assess the preliminary activity of DLYE5953A in metastatic breast cancer (HER2-negative MBC) and NSCLC.

The study was conducted in accordance with the principles of the Declaration of Helsinki and Good Clinical Practice. Approval from the Institutional Review Boards and ethics committees was obtained before study start. Written-informed consent was obtained from patients prior to enrollment. The study was registered on ClinicalTrials.gov (NCT02092792).

Key eligibility criteria for patients included age ≥18 years, Eastern Cooperative Oncology Group (ECOG) performance status of 0–1, histologically or cytologically documented advanced or metastatic breast, ovarian, pancreatic, NSCLC, head and neck squamous cell carcinoma, or gastric cancer in the dose-escalation cohort, adequate hematologic and end organ function, and measurable disease per RECIST v1.1. In addition, tissue samples for assessment of LY6E expression were required of patients dosed at ≥1.6 mg/kg DLYE5953A. There were no eligibility restrictions with regards to prior therapies in the dose-escalation portion of the study, except that patients who had received treatment with another ADC containing MMAE were excluded.

The dose-expansion cohorts included patients with HER2-negative MBC and NSCLC who had received ≤2 prior lines of chemotherapy in the advanced and/or metastatic setting. Eligibility was not limited with regards to the number of prior nonchemotherapeutic agents (e.g., endocrine therapy, CDK4/6 inhibitors, mTOR inhibitors). Patients with HER2-negative MBC were required to have received a taxane and/or anthracycline and at least one cytotoxic regimen in the metastatic setting. Patients with EGFR mutation–positive NSCLC or ALK rearrangement–positive NSCLC were required to have received an EGFR inhibitor or an ALK inhibitor, respectively. Patients with grade ≥2 toxicity from prior therapy or grade ≥2 peripheral neuropathy regardless of causality were excluded.

All patients who received DLYE5953A were assessed for safety based on reports of adverse events (AEs), clinical laboratory testing, vital signs, physical examination, and electrocardiography. AEs were graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events, v4.0. Infusion-related reactions (IRR) were defined as any infusion-related sign/symptom that occurred during or within 24 hours of DLYE5953A infusion. AEs meeting criteria of as an IRR were reported and graded as individual signs and symptoms rather than as an IRR, and grade ≥2 AEs related to an IRR were considered as an AE of special interest. A dose-limiting toxicity (DLT) was defined as any grade ≥3 nonhematologic study drug–related AE occurring within the first 21 days with certain exceptions including grade 3 nausea, vomiting, or diarrhea that resolved to grade ≤1 within 7 days and IRRs that did not limit retreatment (Supplementary Tables S1 and S2; complete definition of DLTs and AEs of special interest). The MTD was defined as the highest dose level at which less than one third of a minimum of six patients experienced a DLT.

Samples for PK evaluation were collected at multiple time points following the first dose of DLYE5953A and less frequently in subsequent cycles. Concentration–time profiles were determined by dose level using a validated ELISA for DLYE5953A with a minimum quantifiable concentration in serum of 50 ng/mL. Unconjugated MMAE analyte concentrations in plasma were determined using a validated LC/MS-MS with electrospray ionization assay, with a lower limit of quantitation of 0.036 ng/mL. PK parameters for total antibody and unconjugated MMAE were determined using noncompartmental approach based on concentration–time profile in cycle 1.

For immunogenicity assessments, a validated antibody bridging ELISA (8) was used to detect ADA formation, characterize the ADA domain specificity, and determine the titer of confirmed ADAs in patient samples.

Disease status was assessed using the RECIST version 1.1. Patients underwent tumor assessments at screening, during the final week of every even numbered cycle until cycle 8 and every three cycles thereafter, and at study discontinuation. Best overall response, objective response, and time on study were evaluated by indication and dose level. Objective response was defined as a complete response or partial response (PR) that was confirmed ≥4 weeks after initial documentation of response. Time on treatment was defined as time from first treatment dose to 21 days after the last dose of DLYE5953A or date of death, whichever came first. Patients with no postbaseline tumor response assessment were considered nonresponders.

Given that LY6E is expressed on subsets of human peripheral blood leukocytes (Genentech, data on file), blood samples were collected to monitor circulating T cells, B cells, and natural killer (NK) cells (TBNK) prior to DLYE5953A administration on day 1 of cycles 1, 5, and 9, and every four cycles thereafter, and at discontinuation of study participation. Quantification of cells in blood samples was performed at a central lab using a standard BD Biosciences TBNK flow cytometry panel including markers for B cells (CD19), T cells (CD3), T helper cells (CD4), cytotoxic T cells (CD8), and NK cells (CD16/CD56).

Tumor specimens, including baseline (either archival or biopsy performed during screening) and on-treatment biopsies, were used to assess LY6E expression using an IHC-based assay, as well as by qRT-PCR. Selected patients who had consented for the procedure underwent a progression biopsy at the end of treatment. IHC for LY6E was performed using a mouse monoclonal IgG1 antibody (clone 15A5; Genentech, Inc.) with automated staining on the BenchMark XT system (Ventana Medical Systems) as previously described (1). Tissues with no detectable signal in >50% of tumor cells were given an IHC score of 0. Tumors with >50% of cells staining for LY6E with the majority of cells staining weakly, moderately, or strongly were given an IHC score of 1+, 2+, or 3+, respectively. The percentage of cells with LY6E staining was also recorded. H-scores (range, 0–300) were calculated as the summation of the percentage of cells at each staining intensity level (0, 1, 2, and 3) multiplied by the respective staining intensity level.

For LY6E qRT-PCR, mRNA was isolated from four formalin-fixed paraffin-embedded (FFPE) tumor sections (whole slide scrapes where tumor content ≥50%, macrodissected tumor epithelium where tumor content <50%) using Roche HighPure FFPET RNA isolation kit (Roche Diagnostics Corporation) according to the manufacturer's instructions. Fifty nanogram total RNA was tested using a Roche LY6E-specific Cobas qRT-PCR assay. Relative expression of LY6E was expressed as ∂C_p. Exploratory whole transcriptome RNA sequencing analysis was also performed on total RNA isolated from samples collected at baseline and at time of study discontinuation, if available.

This study was intended to obtain descriptive preliminary safety, PK, and activity information in the treated populations, and as such, sample sizes did not reflect explicit power and type I error considerations. Patients who withdrew from the study prior to completing the DLT assessment window (cycle 1, days 1–21) for reasons other than DLT were considered nonevaluable for DLT and MTD assessments. All analyses were based on the safety-evaluable population and presented according to the assigned dose level. All statistical analyses were carried out in SAS 9.2 and R 3.1.1. software.

Between April 2014 and July 2017, a total of 69 patients with solid tumors including breast, NSCLC, ovarian, and pancreatic cancers were enrolled at five centers in the United States, 68 of whom received ≥1 dose of DLYE5953A and were therefore safety evaluable. Demographics for patients with HER2-negative MBC and NSCLC dosed at the RP2D are provided (Table 1); disease characteristics by dose level are provided for patients who participated in the dose-escalation stage in Supplementary Table S3. Overall, the median patient age was 58 (range, 33–82 years). For NSCLC, 52% of the patients were female and 48% were male, whereas in MBC, 97% were female and 3% were male. At the RP2D (n = 29 MBC and n = 25 NSCLC), 32% of patients with NSCLC had squamous cell carcinomas, whereas 68% were nonsquamous; 28% of patients with MBC had TNBC and 72% had HR-positive/HER2-negative MBC. Of the patients who were treated with DLYE5953A at 24 mg/kg and whose tumors were evaluable for LY6E (n = 46/54, 85%), 63% were from the primary tumor and 35% were from a metastatic lesion; one sample (2%) was of unknown origin. A higher percentage of patients with MBC (26/29; 90%) treated at the RP2D of 2.4 mg/kg had been previously treated with a taxane than patients with NSCLC (16/25; 64%). In this same subset of patients, only two patients (7%) with MBC had been previously treated with a checkpoint inhibitor, whereas all but three of patients with NSCLC (88%) had received prior immune checkpoint blockade. Prior platinum-based therapy included 31% for patients with MBC (9/29) and 100% (25/25) for patients with NSCLC. Analyses of tumor tissue samples by IHC showed high (2/3+) LY6E expression in 74% of patients with HER2-negative MBC and 76% of patients with NSCLC enrolled in this study.

Safety-evaluable patients (n = 68) received a median of four cycles (range, 1–27) of DLYE5953A at doses ranging from 0.2 to 2.4 mg/kg IV Q3W, including 10 of 68 (15%) patients who remained on study treatment for over 6 months. Twenty patients received DLYE5953A in dose-escalation cohorts at 0.2 (n = 3), 0.4 (n = 3), 0.8 (n = 3), 1.6 (n = 4), and 2.4 (n = 7) mg/kg (Supplementary Fig. S1). Fifty-five patients received DLYE5953A at the RP2D of 2.4 mg/kg, including patients with MBC (n = 29; n = 23 from expansion and n = 6 from escalation), patients with NSCLC (n = 25 from expansion), and one patient in the dose-escalation group with ovarian cancer. Time on treatment is presented for all patients with MBC and NSCLC treated at 2.4 mg/kg (Fig. 1) and for patients treated at doses below the RP2D of 2.4 mg/kg or with indications other than NSCLC or MBC in Supplementary Fig. S2. Patients with MBC who were treated at the RP2D of DLYE5953A (n = 29) were on study treatment for a median of four cycles (1–20) or 82 (22–471) days, whereas patients with NSCLC (n = 25) remained on study for a median of four cycles (1–27) or 87 (10–611) days (Fig. 1).

No DLTs were reported in the study. MTD was not reached, and RP2D for DLYE5953A was determined to be 2.4 mg/kg Q3W. The rationale for stopping dose escalation at 2.4 mg/kg was based on the totality of safety data, including tolerability beyond cycle 1 at 2.4 mg/kg Q3W dosing. Two out of six patients in the dose-escalation cohort who were treated beyond cycle 1 with 2.4 mg/kg Q3W required dose reductions for AEs, suggesting higher doses of DLYE5953A were unlikely to be tolerated. Overall, six of 68 patients (9%) discontinued treatment with DLYE5953A due to an AE, of which five were considered related to study drug, all at doses of either 1.6 or 2.4 mg/kg. The only AE that led to study drug discontinuation in more than one patient was peripheral neuropathy (n = 1 grade 3, n = 2 grade 2). All other AEs that led to treatment discontinuation were reported in one patient each: neutropenia with port infection (grade 3); abdominal pain and vomiting (both grade 3); and pleural effusion (grade 5). Dose modifications (interruptions, delays, or reductions) were limited to patients initially treated at the 1.6 or 2.4 mg/kg dose levels. Five patients had a total of seven AEs leading to dose reduction, including two patients with grade 3 events. Seven patients had a total of 11 AEs leading to dose interruption, all of which were signs and symptoms of IRRs of grade 1 (three AEs), grade 2 (seven AEs), and grade 3 (one AE).

The most common treatment-related AEs, defined as occurring in ≥20% (17/68) patients at the RP2D, were alopecia (53%), fatigue (46%), nausea (38%), peripheral neuropathy (32%; includes peripheral neuropathy, peripheral sensory neuropathy, peripheral motor neuropathy, paresthesia, hypoesthesia, neuralgia, and muscular weakness), chills (28%), decreased appetite (28%), and diarrhea (21%; Table 2). Grade ≥3 related AEs (Supplementary Table S4) occurred in 17 of 68 (25%) of patients, with neutropenia being the most common (7/58; 10%), including one patient with a serious AE (SAE) of neutropenia and port infection leading to DLYE5953A discontinuation (noted above). However, no dose reductions occurred due to neutropenia, which was managed with dose delays and growth factor administration. Twelve patients (12/68, 18%) received growth factor support. All AEs that occurred at ≥10% frequency regardless of attribution and grade ≥3 AEs regardless of attribution are summarized in Supplementary Tables S5 and S6, respectively. Summaries of all and related AEs by dose for patients who participated in dose escalation are provided in Supplementary Tables S7 and S8, respectively.

Overall, 18 of 68 (27%) patients experienced SAEs. Five of these patients experienced SAEs that were assessed as related to DLYE5953A (Supplementary Table S9; n = 1 nausea and pain in extremity; n = 1 peripheral neuropathy; n = 1 device-related port infection and neutropenia; n = 1 abdominal pain, constipation, and vomiting; n = 1 supraventricular tachycardia and ileus). Four deaths were reported, none considered related to DLYE5953A. Two patients in the dose-escalation stage died due to disease progression. The other two patients died from AEs, including one patient with ovarian cancer with suspected bacterial peritonitis and pleural effusion, and another with triple-negative breast cancer with lung, bone, and brain metastases who had respiratory failure.

Overall, grade ≥2 AEs related to IRRs were reported in 15 (22%) patients with a single AE being a grade ≥3 AE (grade 3 hypertension). Chills were the most frequently reported IRR symptoms and were managed with meperidine as needed. Starting at the 2.4 mg/kg dose level, patients were recommended to receive prophylactic therapy with diphenhydramine and acetaminophen. If an IRR was experienced despite this prophylactic regimen, the patient was generally pretreated with steroids prior to subsequent DLYE5953A infusions. At 2.4 mg/kg dosing, one of the four patients who did not receive premedications had a grade ≥2 IRR-related AE, and eight of 51 patients who were reported to have received premedications had such an event. No patients discontinued or were dose reduced due to IRRs; however, DLYE5953A dosing was interrupted in seven patients due to IRRs (median time protocol-defined infusion time was exceeded: 45 minutes; range, 0–140 minutes).

Thirty-seven of 68 (54%) patients developed alopecia, which included grade 1 (n = 20) and grade 2 (n = 17) events. Hair loss in some patients involved eyelashes and eyebrows. All but two patients who experienced alopecia concurrently experienced ocular AEs; hair loss preceded the AE related to eye disorder in all cases. Two patients experienced ocular AEs but did not experience alopecia: one patient presented with vitreous floaters, and the other, dry eyes.

Peripheral neuropathy is an AE commonly reported with tubulin inhibitors such as MMAE (9). Nineteen of 55 (35%) patients at the RP2D experienced peripheral neuropathy that was considered related to DLYE5953A, presenting more often in patients with MBC (13/29; 45%) than in patients with NSCLC (6/25; 24%). Peripheral motor neuropathy was only reported in patients with MBC (8/29, 28%). Most events of peripheral neuropathy were grade 1, although four patients with MBC experienced grade 2 (n = 2) or grade 3 (n = 2) events and one patient with NSCLC experienced one grade 3 event. Median time to onset for patients who experienced related grade ≥ 2 peripheral neuropathy was 160 days. Three patients, all treated at the RP2D of 2.4 mg/kg, discontinued DLY5953A due to peripheral neuropathy. Peripheral neuropathy recurred after initial resolution in nine of 13 (69%) of patients with MBC treated at the RP2D; none of the patients with NSCLC experienced recurrent events of peripheral neuropathy. Every patient who experienced recurrent events of peripheral neuropathy also had a history of peripheral neuropathy prior to study enrollment. Time of initial onset for peripheral neuropathy while on study ranged from as early as study day 3 in a patient with MBC to as late as study day 169 in a patient with NSCLC. The majority of peripheral neuropathy AEs was reported as resolved or resolving at the end of the reporting period for the study, which ended 30 days after a patient's last dose.

All 68 patients who received at least one dose of DLYE5953A were evaluable for PK analyses. Systemic exposure of DLYE5953A increased with increasing dose (Supplementary Fig. S3). Total antibody AUC_0-inf was dose proportional at doses ≥0.8 mg/kg (Table 3). At 2.4 mg/kg of DLYE5953A, clearance was 10.7± 3.6 mL/day/kg, and the half-life was 7.21 ± 2.35 days. The C_max of unconjugated MMAE at the 2.4 mg/kg dose of DLYE5953A was 4.77 ± 2.3 ng/mL. There was no difference in PK between patients with MBC and NSCLC.

The postbaseline incidence of ADAs was 25% (16 treatment-emergent ADAs out of 64 postbaseline evaluable patients). Formation of ADAs appeared to be related to increased clearance of DLYE5953A total antibody in two patients; however, no apparent effect of ADAs on the PK was observed in the other 14 patients with treatment-emergent ADAs. Partial responses were observed in the presence of ADAs in five patients including one patient for whom ADAs appeared to have increased DLYE5953A total antibody clearance.

All of the 68 safety-evaluable patients were evaluable for efficacy, including seven patients with no postbaseline radiographic tumor response assessment (five at the RP2D of 2.4 mg/kg and two in dose escalation) who were considered nonresponders. Thirty-seven of 68 (54%) patients had stable disease as their best response (Supplementary Fig. S4). The confirmed overall objective response rate was 12%. All responders (8/68 patients) had PR, and all had received the RP2D dose of 2.4 mg/kg (Fig. 2). This included two patients with MBC in the 2.4 mg/kg dose-escalation cohort, one patient in the MBC expansion cohort (MBC: 3/29; 10%), and five patients in the NSCLC expansion cohort (5/25; 20%). Six of the eight patients with confirmed partial response had tumors with LY6E IHC scores of 2+ or 3+.

One of the patients who experienced a PR was a 64-year-old female diagnosed with NSCLC with an LY6E IHC score of 3+ (Fig. 3A). This patient had progressive disease after successive treatments with cisplatin/pemetrexed (neoadjuvant setting), carboplatin/pemetrexed, and nivolumab. She was treated with DLYE5953A at 2.4 mg/kg and experienced a PR at cycle 2 that was confirmed at cycle 4 at −46%. She was discontinued from study after cycle 6 due to growth of a new lesion. Another patient diagnosed with TNBC experienced nearly complete amelioration of skin lesions (Fig. 3B). This patient was a 46-year-old female with an LY6E IHC score of 3+. She had previously received cyclophosphamide/adriamycin/taxol (adjuvant setting), cisplatin, and eribulin/pembrolizumab. She was treated with DLYE5953A at 2.4 mg/kg and achieved a best radiographic response of -14% at the end of cycle 2. Her cycle 4 scans indicated regrowth of target lesions (increase of 13% from cycle 2), and she withdrew from study following cycle 5 to undergo a mastectomy.

There was a dose-dependent decline in peripheral NK cells (Supplementary Fig. S5), but no evidence of decline in T cells or B cells compared with pretreatment baseline levels with DLYE5953A treatment (data not shown). For most patients, NK levels remained within the normal range. No unusual/opportunistic infections were reported in patients who experienced a decline in peripheral NK cells.

Three patients, all with MBC, provided postprogression biopsies (Supplementary Fig. S6), including the patient featured in Fig. 3B (patient 2 in Supplementary Fig. S6). IHC scores for LY6E changed minimally over treatment with DLYE5953A, and H-score did not decline >15% for any of these three patients. However, the multidrug resistance (MDR) pump, ABCB1, was expressed in all three postprogression tumor biopsies and was increased compared with pretreatment levels in the two patients with available pretreatment tumor RNA samples. ABCC2 was likewise increased in the postprogression biopsy in the two patients with pre/post RNA samples.

Results from this phase I study demonstrate that DLYE5953A, an anti-LY6E ADC, has an acceptable safety profile when administered by i.v. infusion Q3W to patients with solid tumors, including MBC and NSCLC. No DLTs were identified for DLYE5953A in dose-escalation cohorts up to 2.4 mg/kg Q3W, which was the maximum administered dose. Although a protocol-defined MTD was not reached, the RP2D for DLYE5953A was identified to be 2.4 mg/kg Q3W, which has been selected for several other ADCs delivering MMAE with similar drug–antibody ratios (10, 11). DLYE5953A at the RP2D had an acceptable safety and tolerability profile with manageable AEs. The overall response rate was 12% with all responses occurring at the RP2D [5/25 (20%) in NSCLC and 3/29 (10%) in MBC]. The safety profile of DLYE5953A was in line with previous reports for ADCs using MMAE as the cytotoxic agent (12).

However, notable differences in the safety profile of DLY5953A include rates of alopecia and IRRs. Alopecia was reported in 54% of all patients, a rate approximately two- to threefold higher than other vc-MMAE–containing ADCs (12–14). Alopecia may be a target-mediated effect given the expression of LY6E in skin (15) and hair follicles (16), although this study has the limitations of a small phase I trial and the lack of a comparator. DLYE5953A treatment also caused IRRs in some patients, all of which were nonserious and clinically manageable. IRRs may also be a target-mediated effect because LY6E is expressed on several different populations of immune cells, including NK cells, and to a lesser extent, T and B cells (1, 2, 4, 17). Reductions in peripheral blood NK cells in some patients treated with DLYE5953A at 2.4 mg/kg further suggest that targeting LY6E may modulate immune function.

Interestingly, the frequency of some AEs was higher in patients with MBC treated at the RP2D in comparison with patients with NSCLC, despite a similar overall time on treatment between the two indications. Particularly notable was the higher frequency of peripheral neuropathy reported in patients with MBC at 2.4 mg/kg compared with patients with NSCLC, possibly explained by their higher prior exposure to taxanes (18). Overall, however, the toxicity profile in this phase I study of DLYE5953A appears to be mainly driven by the MMAE component, and exploration of other cytotoxic payloads conjugated to anti-LY6E antibodies may yield an ADC with a more favorable therapeutic index.

PK of DLYE5953A was linear at ≥0.8 mg/kg. The PK of total antibody and unconjugated MMAE appear to be comparable with other vcMMAE ADCs that have been evaluated clinically (6, 12, 13). Minimal accumulation of total antibody for a Q3W dosing regimen was expected based on the terminal t_1/2 of approximately 7 days at 2.4 mg/kg. The postbaseline incidence of ADAs was 25%. Formation of ADAs appeared to be related to increased clearance of DLYE5953A total antibody in two patients; however, no apparent effect of ADAs on the PK was observed in the other 14 patients with treatment-emergent ADAs. Partial responses were observed in the presence of ADAs in five patients including one patient for whom ADAs appeared to have increased DLYE5953A total antibody clearance.

Antitumor activity was observed in patients whose tumor LY6E IHC scores were 2+ or 3+ in patients with HER2-negative MBC and patients with NSCLC, all at the RP2D. Although the majority of patients treated with DLYE5953A had IHC scores of 2 or 3, objective antitumor activity within this patient population varied widely (from +31% to −72%). In patients with NSCLC, there was no correlation between prior cancer immunotherapy and overall response.

Possible mechanisms that might explain acquired resistance to DLYE5953A include a loss of target expression (i.e., LY6E) by the tumor cells, failure of LY6E to be internalized, resistance to MMAE-induced apoptosis, and/or upregulation of MDR pumps. In the three patients who provided both pretreatment and progression tumor biopsies, there was no indication that LY6E expression declined appreciably. However, the MDR pump ABCB1 (or PgP), which is a known efflux pump for MMAE (19), was increased in the two samples for which pre- and posttreatment RNA samples were available. In addition, ABCC2 (or MRP2), also a known MDR pump expressed highly in liver (20), was expressed in all the postprogression biopsies and was increased in the two paired samples. These findings suggest that induction of MDR pumps may partially contribute to acquired resistance to DLYE5953A (21).

In conclusion, DLYE5953A demonstrated antitumor activity in patients with locally advanced or metastatic HER2-negative MBC and NSCLC, a population with relatively poor prognosis. Patients with evaluable tumor tissue and an objective response to treatment had LY6E IHC scores of 2+ or 3+. These findings provide clinical rationale for developing LY6E-directed therapies for patients with malignancies that overexpress LY6E, in particular HER2-negative MBC and NSCLC. Although the safety profile of DLYE5953A was acceptable, use of a cytotoxic agent other than MMAE may improve the therapeutic index of an ADC targeted against LY6E.

S.M. Tolaney reports grants from Genentech/Roche (research funding to institution) during the conduct of the study; grants and personal fees from AstraZeneca (research funding to institution; honorarium for consulting/advisory board), Eli Lilly (research funding to institution; honorarium for consulting/advisory board), Merck (research funding to institution; honorarium for consulting/advisory board), Nektar (research funding to institution; honorarium for consulting/advisory board; travel expense reimbursement), Novartis (research funding to institution; honorarium for consulting/advisory board), Pfizer (research funding to institution; honorarium for consulting/advisory board), Genentech/Immunomedics (research funding to institution; honorarium for consulting/advisory board (outside this study)), Immunomedics (research funding to institution; honorarium for consulting/advisory board), grants from Exelisis (research funding to institution), grants and personal fees from Bristol-Myers Squibb (research funding to institution; honorarium for consulting/advisory board), Eisai (research funding to institution; honorarium for consulting/advisory board), Nanostring (research funding to institution; honorarium for consulting/advisory board), personal fees from Puma (honorarium for consulting/advisory board), grants from Cyclacel (research funding to institution), grants and personal fees from Sanofi (research funding to institution; honorarium for consulting/advisory board), personal fees from Celldex (honorarium for consulting/advisory board), grants and personal fees from Odonate (research funding to institution; honorarium for consulting/advisory board), personal fees from Seattle Genetics (honorarium for consulting/advisory board), Daiichi Sankyo (honorarium for consulting/advisory board), Silverback Therapeutics (honorarium for consulting/advisory board), Athenex (honorarium for consulting/advisory board), Abbvie (honorarium for consulting/advisory board), OncoPep (honorarium for consulting/advisory board), Kyowa Kirin Pharmaceuticals (honorarium for consulting/advisory board), and Samsung Bioepis Inc. (honorarium for consulting/advisory board) outside the submitted work. K.T. Do reports personal fees from Jackson Laboratories and personal fees from QED Therapeutics outside the submitted work. P.M. LoRusso reports personal fees from AbbVie (Advisory Board (2018–2019)), Agios (Data Safety Monitoring Board (2016–2019)), Five Prime (Data Safety Monitoring Board (2017–2020)), GenMab (Advisory Board Member (2016–2019)), Halozyme (Data Safety Monitoring Board (2016–2019)), Genentech (Advisory Board (2016–2019)), CytomX (Advisory Board (2016–2019)), Takeda (Advisory Board (2017–2020)), Sotio (Consultant (2018–2019)), Cybrexa (Advisory Board (2018–2019)), Agenus (Advisory Board (2018–2020)), Tyme (Data Safety Monitoring Board (2018–2020)), IQVIA (Advisory Board (2019–2020)), TRIGR (Advisory Board (2019–2020)), Pfizer (Advisory Board (2019–2020)), I-MAB (Advisory Board (2019–2020)), ImmunoMet (Advisory Board (2018–2020)), Black Diamond (Advisory Board (2019–2020)), Glaxo-Smith Kline (Advisory Board (2019–2020)), QED Therapeutics (Advisory Board (2019–2020)), AstraZeneca (Advisory Board (2019–2020)), EMD Serono (Advisory Board (2019–2020)), Shattuck (Advisory Board (2019–2020)), Astellas (Advisory Board (2019–2020)), Salarius (Advisory Board (2019–2020)), Silverback (Advisory Board (2019–2020)), MacroGenics (Advisory Board (2019–2020)), Kyowa Kirin (Advisory Board (2020–2021)), Kineta (Advisory Board (2020–2021)), Zentalis (Advisory Board (2020–2021)), and Molecular Templates (Advisory Board (2020–2021)) outside the submitted work. C.D. Weekes reports other from Genentech (research support) during the conduct of the study, as well as other from Genentech (research support) outside the submitted work. S. Chandarlapaty reports other from MSKCC/Genentech (clinical trial funding) during the conduct of the study; personal fees and other from Novartis (clinical trial funding to institution and consulting fees to individual) and Eli Lilly (clinical trial funding to institution and consulting fees to individual), personal fees from BMS, Sermonix, Context Therapeutics, other from Sanofi (clinical trial funding to institution), and personal fees from Revolution Medicine and Paige.ai outside the submitted work. C.W. Chang reports employment and stock options from Genentech/Roche. S-C. Chen reports other from Roche (employee and stock holder) and other from Retrophin (employee and stock holder) outside the submitted work. D. Nazzal reports employment and stock options from Genentech/Roche. F. Brunstein reports other from Genentech/Roche (employee of Genentech/Roche and stockholder in Roche) outside the submitted work. M. Carrasco-Triguero reports other from Genentech (employee) outside the submitted work. W.C. Darbonne reports personal fees from Genentech/Roche (employment) during the conduct of the study, as well as personal fees from Genentech/Roche (employment) outside the submitted work. J.M. Giltnane reports other from Genentech (employee) during the conduct of the study; other from Genentech (employee) outside the submitted work; and that she is an employee of Genentech. W.M. Flanagan reports other from Genentech (stockholder) during the conduct of the study. S.R. Commerford reports being an employee at Roche/Genentech and stockholder at Roche. A. Ungewickell reports being an employee and stockholder of Genentech/Roche, and a stockholder of Agios, Alexion Pharmaceuticals, Alnylam, Amicus Therapeutics, Bristol-Myers Squibb, Regeneron Pharmaceuticals, Rosetta Genomics, and Seattle Genetics. G.I. Shapiro reports other from Genentech (funding to Dana-Farber for the conduct of the trial) during the conduct of the study; grants and personal fees from Lilly (Sponsored Research Agreement and Scientific Advisory Board), Merck KGaA/EMD Serono (Sponsored Research Agreement and Scientific Advisory Board), grants from Merck & Co. (Sponsored Research Agreement), grants and personal fees from Sierra Oncology (Sponsored Research Agreement), personal fees from Pfizer (Scientific Advisory Board), G1 Therapeutics (Scientific Advisory Board), Roche (Scientific Advisory Board), Bicycle Therapeutics (Scientific Advisory Board), Fusion Pharmaceuticals (Scientific Advisory Board), Cybrexa Therapeutics (Scientific Advisory Board), Astex (Scientific Advisory Board), Almac (Scientific Advisory Board), Ipsen (Scientific Advisory Board), Bayer (Scientific Advisory Board), Angiex (Scientific Advisory Board), Daiichi Sankyo (Scientific Advisory Board), Seattle Genetics (Scientific Advisory Board), Boehringer Ingelheim (Scientific Advisory Board), ImmunoMet (Scientific Advisory Board), Asana (Scientific Advisory Board), Artios (Scientific Advisory Board), Atrin (Scientific Advisory Board), Concarlo Holdings (Scientific Advisory Board), Synta (Scientific Advisory Board), and Zentalis (Scientific Advisory Board) outside the submitted work; in addition, Dr. Shapiro has a patent for Dosage regimen for sapacitabine and seliciclib issued and a patent for Compositions and methods for predicting response and resistance to CDK4/6 inhibition pending. S. Modi reports other from Genentech (Research Funding, Advisory/consulting, Speakers Bureau) during the conduct of the study, as well as other from Daiichi Sankyo (Research Funding, Advisory/consulting, Speakers Bureau), Astra Zeneca (Research Funding, Advisory/consulting, Speakers Bureau), Seattle Genetics (Research Funding, Advisory/consulting, Speakers Bureau), Novartis (Research Funding, Advisory/consulting), and Macrogenics (Advisory/consulting) outside the submitted work. No potential conflicts of interest were disclosed by the other authors.

The authors take full responsibility for the design of the study, the collection of the data, the analysis and interpretation of the data, the decision to submit the article for publication, and the writing of the article.

S.M. Tolaney: Supervision, investigation, writing-review and editing. K.T. Do: Supervision, investigation, writing-review and editing. J.P. Eder: Investigation, writing-review and editing. P.M. LoRusso: Investigation, writing-review and editing. C.D. Weekes: Investigation, writing-review and editing. S. Chandarlapaty: Investigation, writing-review and editing. C.-W. Chang: Data curation, formal analysis, writing-review and editing. S.-C. Chen: Data curation, formal analysis, writing-review and editing. D. Nazzal: Investigation, writing-review and editing. E. Schuth: Investigation, writing-review and editing. F. Brunstein: Formal analysis, writing-review and editing. M. Carrasco-Triguero: Investigation, writing-review and editing. W.C. Darbonne: Data curation, methodology, writing-review and editing. J.M. Giltnane: Investigation, writing-review and editing. W.M. Flanagan: Conceptualization, supervision, writing-review and editing. S.R. Commerford: Supervision, writing-original draft, writing-review and editing. A. Ungewickell: Supervision, writing-original draft, writing-review and editing. G.I. Shapiro: Supervision, investigation, writing-review and editing. S. Modi: Investigation, writing-review and editing.

The authors wish to thank all the patients and their families, and the investigators who participated in this study. Editing and writing support was provided by A. Daisy Goodrich (Genentech, Inc.) and was funded by Genentech, Inc. The authors would like to thank gCELL, Genentech LAR, the Genentech pCORE laboratories and computational biology group, and Elizabeth Lu for their help in various aspects of this work. S. Tolaney, K.T. Do, and G.I. Shapiro were supported by National Cancer Institute - Cancer Center Support Grant (NCI-CCSG) P30 CA006516. S. Chandarlapaty and S. Modi were supported by NCI-CCSG P30 CA008748.

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