Early Signs of Tolerability and Activity in a Phase 2 Study of Weekly Vincristine Sulfate Liposomal Injection (VSLI, MarqiboÒ) in Adults with Philadelphia Chromosome-Negative Acute Lymphoblastic Leukemia (ALL) in Second Relapse or Progressing Following Two Anti-Leukemia Treatment Lines

Increased drug delivery through enhanced formulations of standard acute lymphoblastic leukemia (ALL) therapies may improve outcomes while producing less toxicity. Preclinical and Phase 1 and Phase 2 clinical studies of vincristine sulfate encapsulated in liposomes (VSLI, Marqibo^®) have shown enhanced efficacy and acceptable tolerability in a variety of solid and hematologic malignancies. A maximum tolerated dose (MTD) of 2.25 mg/m² with no dose cap has been established for VSLI, which contrasts with the 2 mg dose cap for conventional vincristine (VCR). Pharmacokinetic studies have shown that altered distribution and elimination phases may lead to increased exposure versus traditional VCR and may account for the increased efficacy observed in nonclinical models. The current Phase 2 trial is evaluating VSLI in adult (Ph-) subjects with ALL in second relapse or after progression following two prior ALL therapies. Subjects are receiving intravenous (IV) 2.25 mg/m² VSLI weekly with no dose cap for up to 12 months. The study will enroll approximately 56 subjects with response rate as the primary endpoint. To date, 23 subjects have received at least one dose of IV VSLI. Preliminary reports indicate that five of these subjects achieved responses (CR/CRi/CRp). While data review is not complete, preliminary signs of efficacy in this underserved patient population warrant discussion:

Two subjects had fewer than 5% lymphoblasts; one on Day 28 and the other on Days 28 and 56 after initial VSLI treatment and subsequently underwent stem cell transplant. The latter subject achieved a CR to a single agent (VSLI) in her first relapse (systemic and extramedullary) after a prior allogeneic transplant.

One subject achieved a CR with no residual blasts (normocellular bone marrow) and resolution of extramedullary disease including resolution of bilateral pleural effusions without thoracentesis and marked decrease of an anterior mediastinal mass by Day 28. Her chest pain resolved after the first dose of VSLI. This subject subsequently went on to receive a second stem cell transplant.

One subject with extramedullary disease of the kidney achieved a CR with no residual blasts detected in the kidney or bone marrow. This subject achieved a true CR (2 confirmed kidney biopsies one month apart) after 11 doses VSLI.

One subject with bone marrow disease achieved a CR that was durable for approximately 5 months.

Although these data are preliminary in this patient population, these findings are encouraging given that this is a single agent chemotherapy being given to a heavily pretreated leukemic population. The drug appears well tolerated to date with no reports of subjects experiencing Grade 4 toxicity. VSLI appears to have a safety profile similar to conventional VCR. Common AEs include neutropenia, fatigue, constipation, peripheral neuropathy, and nausea. In the previous VSLI (Marqibo) Study VSLI-06, VSLI appeared to be highly effective in second relapsed subjects achieving a CR rate of 29% with a limited sample size (2 out of 7 subjects). The preliminary CR (CR/CRi/CRp) rate from this study further corroborates this result: the CR rate is 28% among patients who completed study treatment and is at least 22% (≥5/23 subjects) among all subjects including those who just started receiving VSLI. The projected rate of activity in this ongoing trial supports these previous Phase 1 study results and confirms the importance of VSLI (Marqibo) in this patient population. These preliminary results are extremely encouraging, as VSLI was given as a single agent to a patient population that typically has a very low response rate to anti-leukemia therapies. Phase 3 combination studies are planned.