The Orally Available Selective Axl Inhibitor BGB324 Induces Diversification of the Immune Repertoire and Specific Changes in Plasma Biomarker Profiles

Introduction: The RTK Axl represents a novel therapeutic target in AML promoting proliferation and therapy resistance. In addition, Axl suppresses immune responses by decreasing antigen presentation and inhibiting NK cells. BGB324 is a first-in-class, orally available, selective Axl kinase inhibitor which is being explored as a therapy for solid tumours and myeloid malignancies. Selective blockade of Axl by BGB324 in AML is currently being explored in the Phase 1/2 trial BGBC003 (NCT02488408). Until now, n=25 patients have been treated in a classical 3+3 dose escalation design and treatment was generally well-tolerated (n=22 with AML and n=3 with MDS). Steady-state levels of BGB324 were achieved between three and six days after initiation of treatment. One AML patient achieved a CRi of five months, a second achieved a PR and another achieved clearance of circulating blasts accompanied by peripheral blood count recovery > three months (ORR 10%). Four additional AML patients (25%) experienced disease stabilisation for more than four months. Two MDS patients experienced a PR.

Here, we report the spectrum of plasma proteins modulated by BGB324 in patients with AML/MDS and their relationship to treatment outcome and compound exposure. Furthermore, we analysed the B- and T- cell repertoire in order to assess potential effects of BGB324 on the immune response in AML patients.

Methods: We used the DiscoveryMap panel (Myriad RBM) to measure plasma protein biomarker levels in a selection of patients pre-dose and at C2D1 and performed a cross-comparison with samples from an ongoing NSCLC study with BGB324 (BGBC004, NCT02424617) to identify candidates specifically modulated in AML. The TCRß repertoire was quantified by Next Generation Sequencing of DNA isolated from peripheral blood MNCs using an Illumina MiSeq sequencer. TCRß gene containing the entire V_ß, D_ß and J_ß segments were amplified with BIOMED2-TCRß-A and -B primer pools. The IGH repertoire was analysed using BIOMED2-FR1/-FR3 primer pools. Using genomic DNA as template, the amplicons were tagged with Illumina adapters and indices in two consecutive PCR reactions. Demultiplexing and FastQ formated data output was generated by the MiSeq reporter. Analysis of TCRß and IGH data was performed on a Microsoft Cloud using our in-house analysis pipeline Pippa, which relies on MiXCR analysis tools.

Results: Treatment with BGB324 significantly modulated Gas6, soluble Axl (sAxl), Angpt2, CXCL1 (GROalpha), SCGB1A1 (uteroglobin, club cell 10kDa protein) and PLAU (uPAR) levels in patient plasma. Of these, Gas6 was modulated after treatment with BGB324 in AML but not in lung cancer patients while CXCL1 and SCGB1A1 were only influenced in lung cancer patients. sAxl levels correlated with BGB324 exposure indicating that the shed Axl receptor represents a novel biomarker of Axl kinase inhibition.

Furthermore, we studied the effects of BGB324 on the T- and B- cell receptor repertoire in n=5 available matched PB samples. We could demonstrate diversification of the TCR repertoire in PB by higher abundance of small T-cell clones at C2D1 with BGB324 compared to pre-treatment levels in three out of five analysed patients.

The analyses of the IGH repertoire in the PB indicates higher diversity at C2D1 in two out of five available patients.

Thus, treatment with BGB324 can increase the diversity of the IGH and TCRb repertoire in the PB of AML patients.

Conclusion: BGB324 is well tolerated in AML patients, exhibits anti-leukemic activity and induces exposure-related modulation of biomarker candidates. Furthermore, BGB324 can induce a diversification of the TCR repertoire in AML patients and might hold potential as an immune-activating drug.