LAM-003, a Novel Oral Heat Shock Protein 90 Inhibitor for Treatment of Acute Myeloid Leukemia, Including Wild-Type and FMS-like Tyrosine Kinase 3 (FLT3)-Mutant Disease

Acute myeloid leukemia (AML) remains a disease with high unmet medical need. While most patients respond to initial therapy, few are cured, relapse rates are high, and most patients eventually develop life-threatening complications. FLT3-mutant disease is a particularly aggressive subtype. Recent approval of drugs targeting FLT3-mutant disease have improved short-term outcomes but not all patients respond, and duration of response is limited by secondary mutations that impede FLT3 inhibitor (FLT3i) binding (intrinsic factors) and by secreted stromal factors that activate alternative pro-survival pathways (extrinsic factors).

Heat-shock protein 90 (HSP90) is a chaperone protein involved in many cellular processes and inhibition of HSP90 can have pleiotropic effects in targeting cancer cells such as degradation of oncoproteins that drive survival and proteins that mediate protective signaling. Here, we describe the nonclinical activity of LAM-003, an orally bioavailable HSP90 inhibitor (HSP90i) under clinical development for AML.

To assess the anti-leukemic activity of LAM-003, we tested a panel of AML cell lines and primary AML samples. LAM-003 inhibited proliferation of both FLT3-mutant and wild-type cell lines, with preferential activity against cells harboring FLT3-ITD (geometric mean FLT3-ITD EC₅₀ = 670 nM [n=8] vs FLT3 WT EC₅₀ = 1400 nM [n=16]). Additionally, we observed that LAM-003 was potent in a subset of the FLT3 WT cells.

To explore whether LAM-003 was effective against tumors driven by oncoproteins that are client proteins of HSP90, we focused on AML cells harboring FLT3-ITD. We confirmed that LAM-003 reduced cell surface FLT3-ITD expression and downstream signaling in MV-4-11 and MOLM-13 cells, consistent with HSP90i-mediated degradation of FLT3-ITD.

In BA/F3 cells expressing FLT3-ITD with various secondary resistance mutations, we observed that LAM-003 elicited a dose-dependent reduction of FLT3 mutant cell surface expression. Moreover, BA/F3 cells expressing FLT3-ITD and the F691L mutation exhibited the expected resistance to crenolanib, yet LAM-003 retained anti-proliferative activity. Additionally, MOLM-13 cells harboring a FLT3 D835Y mutation demonstrated expected resistance to the FLT3i sorafenib and tandutinib yet remained sensitive to LAM-003. Finally, primary AML blasts harboring a D835 mutation displayed sensitivity to LAM-003 when tested ex vivo.

To evaluate the potential of LAM-003 to overcome bone-marrow-stroma-derived resistance, FLT3-ITD AML cell lines (MV-4-11, MOLM-13, MOLM-14) were assayed in unconditioned or stromal-cell-conditioned medium. Conditioned medium dramatically reduced the potency of FLT3i but LAM-003 demonstrated equal potency under both conditions. We also showed that stromal cell co-culture induced FLT3i resistance in MOLM-13 cells whereas LAM-003 retained potent activity.

Recognizing that the inherent genetic heterogeneity of AML blasts limits the curative potential of a single drug, we performed in vitro studies to identify drugs that synergize with LAM-003 in 3 FLT3-ITD AML cell lines. Synergy was demonstrated with FLT3i, daunorubicin, azacitidine or cytarabine, with the most robust synergy being observed with venetoclax. Extending the evaluation to AML cells wild type for FLT3 and cell lines from other hematologic indications (multiple myeloma, diffuse large B-cell lymphoma and mantle cell lymphoma), we found that the synergy was not limited to cells harboring FLT3-ITD, but rather correlated with BCL-2 abundance, suggesting a fundamental mechanism of action that depends on BCL-2 family-mediated survival. Mechanistic studies demonstrated that the combination of LAM-003 and venetoclax inhibited AKT-mediated regulation of GSK3B, resulting in MCL-1 degradation.

In vivo studies using a MOLM-13 systemic model of FLT3-ITD AML demonstrated that LAM-003 monotherapy significantly improved animal survival and that the combination of LAM-003 and venetoclax significantly prolonged animal survival compared with each single agent.

These nonclinical studies demonstrate that LAM-003 exhibits antileukemic activity, overcomes mechanisms of FLT3i resistance and potently synergizes with existing AML drugs. As such, our data provide strong rationale for evaluation of LAM-003 in an ongoing clinical trial in patients with AML (NCT03426605).