ARRY-520 Shows Durable Responses in Patients with Relapsed/Refractory Multiple Myeloma in a Phase 1 Dose-Escalation Study

ARRY-520 is a potent, selective inhibitor of kinesin spindle protein (KSP, eg5) which is required for cell cycle progression through mitosis. Treatment with ARRY-520 arrests cells in mitosis with subsequent induction of apoptosis due to degradation of survival signals during mitotic arrest. Cancers, such as multiple myeloma (MM), that depend on the short-lived survival protein Myeloid cell leukemia (MCL)-1 are highly sensitive to treatment with ARRY-520 in preclinical MM models, providing a strong rationale for its clinical investigation in this disease.

This Phase 1 study was designed to evaluate the safety, pharmacokinetics (PK), preliminary efficacy and biological activity of ARRY-520 administered intravenously on Days 1 and 2 every 2 weeks without/with granulocyte colony-stimulating factor (G-CSF) support. Eligible patients (pts) had relapsed or refractory MM with ≥ 2 prior lines of therapy (including both bortezomib [BTZ] and an immunomodulatory [IMiD] agent), unless refusing or ineligible for this therapy. Cohorts were enrolled in a classical 3+3 dose escalation design.

Enrollment in this Phase 1 study is complete. Thirty-one pts have been treated, with a median age of 60 years (range 43–79) and a median of 6 prior regimens (range 2–16). All pts received a prior proteasome inhibitor (30 pts BTZ, 4 pts carfilzomib) and an IMiD-based agent (28 pts lenalidomide, 23 pts thalidomide). Twenty-four pts had an autologous stem cell transplant.

The maximum tolerated dose (MTD) was determined to be 1.25 mg/m²/day without G-CSF. As neutropenia was the dose-limiting toxicity (DLT), dose escalation with G-CSF support was conducted and the MTD for ARRY-520 with G-CSF was determined to be 1.5 mg/m²/day. At the MTD, 1 of 7 pts had a DLT of febrile neutropenia. At doses above the MTD, additional DLTs of Grade 3 mucositis and Grade 3 corneal disorder were observed.

ARRY-520 demonstrated an acceptable safety profile. The most commonly reported treatment-related adverse events (AEs) included hematologic events (anemia, leukopenia, neutropenia, thrombocytopenia), as well as anorexia, blurred vision, diarrhea, dizziness, fatigue, febrile neutropenia, mucositis, nausea and rash. No treatment-related AEs of neuropathy or alopecia were reported at the MTD. ARRY-520 has been dosed over extended periods of time (to date, median 7 cycles [range 1–44]), with no evidence of cumulative toxicity.

The plasma concentrations of ARRY-520 were determined over a 7-day period during Cycle 1 following the Day 1 and 2 infusions of ARRY-520. The preliminary noncompartmental PK parameter estimates in this population were similar to those observed in prior oncology studies. The PK was characterized by low clearance (CL = 2.2 L/hr/m²) and a large volume of distribution (V_ss = 232 L/m²). The t_1/2 of elimination was very long (67 hrs). Concentrations were typically maintained above the in vitro IC₅₀ for KSP inhibition for ≥ 7 days suggesting therapeutically active concentrations of drug were maintained in pts for sustained periods. Further analyses of PK relative to safety and activity are on-going.

ARRY-520 showed activity as a single agent across a range of doses in this heavily pretreated population (31 evaluable pts) with 3 confirmed partial responses (PR) and 1 confirmed minimal response (MR) per International Melanoma Working Group (IMWG) and European Group for Blood and Marrow Transplantation (EMBT) criteria. PRs had a median of 7 prior therapies (range 2–8). Responses were durable; to date, the durations of responses for PRs were 3.4+ months (mos), 11.9+ mos and 12.0 mos, respectively. Of interest, the time to response with ARRY-520 was prolonged, with a median time to PR of 3.7 mos (range 3.7–8.1). Notably, responses were observed in pts refractory to multiple standard-of-care agents. In addition, 4 pts experienced a best response of stable disease (SD) lasting ≥ 10 mos. To date, 5 pts remain on study, including 2 of 3 PRs.

In this Phase 1 study, ARRY-520 shows promising evidence of clinical activity, with a long duration of response and an acceptable safety profile in heavily pretreated MM Patients. A Phase 2 portion of the study is ongoing to obtain additional information on the efficacy, safety and biological effects of ARRY-520 at 1.5 mg/m²/day with G-CSF support.

Shah:Array BioPharma: Consultancy, Research Funding; Celgene: Consultancy; Onyx: Consultancy, Research Funding. Off Label Use: ARRY-520. Zonder:Millenium: Consultancy, Research Funding, Speakers Bureau; Celgene: Speakers Bureau; Medtronics: Honoraria; Amgen: Consultancy. Cohen:Celgene: Consultancy, Honoraria; Millenium: Consultancy, Honoraria; Onyx: Consultancy, Honoraria. Orlowski:Array BioPharma: Honoraria, Membership on an entity's Board of Directors or advisory committees. Alexanian:Array BioPharma: Research Funding. Thomas:Array BioPharma: Research Funding; Centecor: Research Funding; Novartis: Research Funding; Immunomedics: Research Funding; Celgene: Research Funding; Millenium: Research Funding. Weber:Array BioPharma: Research Funding. Kaufman:Keryx: Consultancy; Celgene: Research Funding; Merck: Research Funding. Walker:Array BioPharma: Employment, Equity Ownership. Litwiler:Array BioPharma: Employment. Karan:Array BioPharma: Employment. Hilder:a: Employment. Ptaszynski:Array BioPharma Inc.: Consultancy. Lonial:Millenium: Consultancy; Celgene: Consultancy; Novartis: Consultancy; Bristol Myers Squibb: Consultancy; Onyx: Consultancy; Merck: Consultancy.