Phase I-IIa Clinical Trial to Assess Safety and Efficacy of MLM-CAR44.1, a CD44v6 Directed CAR-T in Relapsed/Refractory Acute Myeloid Leukemia (AML) and Multiple Myeloma (MM)

Background: The hyaluronate receptor CD44 is a class I membrane glycoprotein overexpressed in hematologic and solid tumors. It plays a role in the bone marrow homing of initiating leukemia cells and in their interactions with the microenvironment. CD44 inhibition drives leukemia cells into differentiation and apoptosis by dislodging them from osteogenic niches (Singh et al, 2013).While CD44 is ubiquitously expressed, the expression pattern of its alternatively spliced isoforms is relatively tumor-restricted. In particular, the isoform variant 6 (CD44v6) has recently emerged as a promising tumor associated antigen. Indeed, it is absent on normal hematopoietic stem cells (HSC) and progenitors, but is expressed on AML and MM, where it correlates with a poor prognosis (Legras et al, 1998; Liebisch et al, 2005). It has been demonstrated that CD44v6 CAR-T cells are able to efficiently kill leukemia and myeloma cells while sparing HSC and keratinocytes, which express low levels of CD44v6 (Casucci et al, 2013; 2018; Norelli et al, 2018). MLM-CAR44.1 is a medicinal product defined as frozen T lymphocytes genetically modified by retroviral transduction to express the CD44v6ΔNL CAR and the herpes simplex virus (HSV)-TK Mut2 suicide gene. This suicide gene was inserted in order to minimize risks of severe toxicities by MLM-CAR44.1. These include predicted monocytopenia and potential cutaneous toxicity, which although not observed in ad hoc mouse model, should be kept under consideration due to CD44v6 expression by keratinocytes. Here we present the first in man phase I-IIa MLM-CAR44.1 clinical protocol we designed to test safety and efficacy of MLM-CAR44.1. This clinical study protocol is Working Package 4 of grant N. 733297 awarded by the European Commission for the action entitled 'EURopean Endeavour for Chimeric Antigen Receptor Therapies - EURE-CART.

Methods: The study is a seamless Phase I/IIa, open-label, multicenter clinical trial which combines Phase I dose escalation based on toxicity with Phase IIa dose expansion based on antitumor activity. Target population are patients with relapse/refractory MM or AML who are fit but not candidate to further treatments with a curative intent. Primary objectives of the study are the MLM CAR44.1 maximum tolerated dose (MTD) and safety (phase I) and efficacy (phase IIa). An adaptative Bayesian Optimal Interval (BOIN) design with cohorts of 3 patients for each indication (AML and MM) was chosen to identify the MTD, based on a target toxicity rate of 30% and an interval of 25% - 35%. This schema was preferred to minimize the decision errors of dose assignment and to safeguard patient ethics and safety by allowing decisions to be made at any time during the study.

Results: Patients will be enrolled if they have relapsed/refractory AML unlikely to benefit of further cytotoxic therapy and not candidate to allogeneic bone marrow transplantation (allo) or MM after at least 4 different prior treatments in 3 treatment lines, or 4 treatments in 2 treatment lines in case of early relapsing patients (relapse in less than 1.5 years) and an adequate organ function. Previous allo is an exclusion criteria. MLM-CAR44.1 will be infused after lymphodepleting chemotherapy. Primary endpoints of safety will be evaluated 30 days after MLM-CAR44.1 infusion. Up to 15 MM and 15 AML patients are expected for completing phase I. Primary endpoint of efficacy will be evaluated at 2 and 3 months for AML and MM, respectively. Pediatric population will start to be included into the trial only after a successful phase I in adults.

Conclusion: Study is expected to open in the 4^th quarter 2018 with first patient MLM-CAR44.1 infusion by the end of 2018. An update on study and enrollment status will be provided during the meeting.