Background: 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 onset of apoptosis due to degradation of survival signals during mitotic arrest. While immunomodulators [IMiDs] and proteasome inhibitors have improved outcomes in MM, patients with MM that is refractory to both bortezomib (BTZ) and thalidomide (THAL) or lenalidomide (LEN) have a poor prognosis with median survival of 9 months. Since ARRY‑520 is a novel agent with a unique mechanism of action (MOA) relative to current standard-of-care (SOC) agents, it might be expected to show activity in patients refractory to other drugs. Preclinically, ARRY‑520 showed activity in BTZ‑refractory models, suggesting prior treatments may not predict patient response to ARRY-520.
Methods: This Phase 2 study was designed to evaluate the efficacy, safety and biological effects of 1.5 mg/m2/day ARRY‑520 administered intravenously on Days 1 and 2 every 2 weeks with granulocyte colony-stimulating factor (G-CSF) support. Eligible patients had relapsed or refractory MM with ≥ 2 prior lines of therapy (including both BTZ and an IMiD), unless refusing or ineligible for this therapy.
Results: Thirty‑two patients have been treated, with a median age of 65 years (range 51-82) and a median of 5 prior regimens (n = 31; range 2‑20). Twenty-eight patients received prior BTZ, 28 patients prior LEN, 17 patients prior THAL and 25 patients had an autologous stem cell transplant. One patient has been lost to follow-up.
ARRY-520 demonstrated an acceptable safety profile, confirming the safety profile observed in the Phase 1 study. The most commonly reported (≥ 10% of patients) treatment‑related adverse events (AEs) included hematologic events such as anemia (11 patients [34%], 4 Grade 3/4 [12%]), neutropenia (11 patients [34%], 9 Grade 3/4 [28%]) and thrombocytopenia (20 patients [63%], 11 Grade 3/4 [34%]), as well as fatigue (4 patients [16%], 2 Grade 1/2 and 2 Grade 3) and mucositis (4 patients [13%], all Grade 1/2). No treatment‑related events of alopecia or neuropathy were reported. One patient discontinued study due to a treatment-related AE of blisters.
ARRY-520 has shown preliminary activity as a single agent in this heavily pretreated population. To date, of 32 evaluable patients, 3 confirmed partial responses (PR) and 2 confirmed minimal responses (MR) have been observed, per International Myeloma Working Group (IMWG) and European Group for Blood and Marrow Transplantation (EMBT) criteria. PRs had a median of 5 prior therapies (range 2‑8). As observed in the Phase 1 study, the time to response with ARRY‑520 was prolonged. Notably, clinical responses have been observed in this study in patients refractory to both LEN and BTZ. To date, in this ongoing study 33% (5/15) of patients with disease refractory to both LEN and BTZ achieved clinical benefit (PR + MR + SD > 4 months). While this trial has been fully recruited, as of July 2011, 8 patients remain on study.
Conclusions: ARRY-520 is a novel agent with a differentiated MOA relative to other myeloma drugs. ARRY‑520 shows promising evidence of single-agent clinical activity and an acceptable safety profile in heavily pretreated patients with MM. Notably, ARRY‑520 has demonstrated activity in patients refractory to both LEN and BTZ, a population with limited treatment options. An expansion cohort is planned in order to evaluate the safety and efficacy of ARRY‑520 in combination with dexamethasone in patients who are refractory to their last myeloma treatment and refractory to prior LEN, BTZ and dexamethasone.