BL-8040, a Peptidic CXCR4 Antagonist, Induces Leukemia Cell Death and Specific Leukemia Cell Mobilization in Relapsed/Refractory Acute Myeloid Leukemia Patients in an Ongoing Phase IIa Clinical Trial

Background: The bone marrow (BM) niche protects Acute Myeloid Leukemia (AML) cells from chemotherapy. BM homing of AML cells is dependent on CXCR4 and its ligand CXCL12, and high levels of CXCR4 expression correlate with poor survival in AML patients. It is postulated that blocking the CXCL12/CXCR4 axis with a potent CXCR4 antagonist will disrupt the interaction of the malignant blasts with the BM and augment the anti-leukemic effect of chemotherapy. BL-8040 (BKT140) is a 14-residue, cyclic, synthetic peptide capped with an aromatic ring that acts as a selective inhibitor of the CXCR4 chemokine receptor. BL-8040 has strong affinity for the CXCR4 receptor and long receptor occupancy, resulting in a prolonged pharmacodynamic effect. BL-8040 has superior mobilization capacity, inverse agonism activity and direct pro-apoptotic activity against leukemia cells. Preclinical studies have shown that in addition to its activity as a mobilizer of hematopoietic cells, BL-8040 exhibits a CXCR4-dependent selective anti-tumor effect against malignant cells. A clinical trial for the treatment of adult relapsed/refractory AML patients using a combination of BL-8040 and cytarabine (Ara-C) is currently ongoing (NCT01838395).

Method: The phase II clinical trial in relapsed/refractory AML patients includes a dose escalation phase (3+3 design) followed by an expansion phase using the highest safe and efficacious dose group. Each patient receives a once daily SC dose of BL-8040 monotherapy on days 1-2 followed by the same dose of BL-8040 plus Ara-C (1.5g/m² for patients ≥60; 3g/m² for patients ≤60) on days 3-7. For each of the BL-8040 tested doses (0.5, 0.75, 1, 1.25 and 1.5 mg/kg) the safety and tolerability, pharmacokinetic profile and clinical responses are being evaluated. Additionally, CXCR4 expression on leukemia cells, receptor occupancy, timing of mobilization of leukemic blasts and stem/progenitor cells and induction of apoptosis are being followed using frequent peripheral blood sampling (PB) and BM aspirates at baseline and on day 3 prior to Ara-C administration.

Results: Preliminary results from the ongoing study suggest that administration of BL-8040, in combination with Ara-C, is safe and well tolerated at all doses tested to date (0.5-1 mg/kg). There have been no DLTs or SAEs and no delays in count recovery attributable to BL-8040. The primary treatment related AE has been a transient injection site reaction. FACS analysis revealed that 2 days of BL-8040 monotherapy triggered substantial mobilization of AML blasts (CD45+Low/CD34+/CD117+/HLA-DR+) from the BM to the PB. Data from the first 9 subjects revealed a 5.14 fold (range 1.0-8.09) median increase in AML blasts in the PB. Moreover, a dramatic decrease in the amount of AML blasts in the BM following 2 days of BL-8040 monotherapy was noted. FACS data from 7 subjects, for which BM aspirate samples were evaluable for analysis, revealed a median decrease of 70.65% [range(-16.32%)-(-87.68%)] in the number of AML blasts (CD45+Low/CD34+/CD117+/HLA-DR+ out of total CD45+/CD34+ normal/progenitor cells) compared with baseline. Importantly, this effect was specific to the AML blasts as the levels of normal/progenitor cells remained stable post BL-8040 monotherapy (5/7 patients). This decrease of BM AML blasts was evident regardless of whether the patient mobilized AML blasts or not. In support of induction of apoptosis by CXCR4 inhibition, an increase in the number of blasts with cleaved caspase3 was ascertained in day 3 BM biopsies from 6/8 patients. Long receptor occupancy (up to 24 hours post dosing) was observed by FACS analysis measuring surface CXCR4 staining. Pharmacokinetics analysis confirmed increase in BL-8040 exposure over doses with a short plasma half-life.

Conclusion: The current data demonstrate that BL-8040 induces mobilization of AML blasts from the BM and has sustained receptor occupancy. In addition, a direct effect on AML blast viability has been observed in samples obtained during BL-8040 monotherapy. Importantly, the data suggest a differential effect of BL-8040 monotherapy on AML blasts vs. normal progenitors. BL-8040 was found to be safe and well tolerated at all doses tested to date. The updated results of the dose escalation phase of this ongoing study will be presented.