Abstract 3975: Target engagement, thrombocytopenia, and efficacy induced by the dual Bcl2/xL inhibitor AZD4320 are quantitatively linked by a PK/PD model in leukemia xenografts

The proteins Bcl2 and Bcl-xL are often up-regulated in cancer, and hold in check the apoptosis that would normally be initiated by accumulation of the BH3-only proteins Bax and Bak in response to genomic dysregulation. AZD4320 potently disrupts that interaction, initiating the apoptotic cascade in Bcl-2 or Bcl-xL-dependent tumors. Because platelets are known to be dependent on Bcl-xL, thrombocytopenia is an expected on-target effect. AZD4320 was administered at dose levels 0.5-10 mg/kg, both intravenously and extravascularly, to immune-compromised mice inoculated subcutaneously with the RS4;11 model of acute lymphocytic leukemia (ALL). Drug concentrations were measured by liquid chromatography-mass spectrometry (LC-MS) in plasma and tumor. A cleaved-caspase-3 ELISA was used to assess apoptotic activity in the tumor. Parallel efficacy studies, were used to assess tumor growth compared to vehicle, following tumors from initial regression at tolerated doses to regrowth. Tumors were measured using calipers, and tumor volumes computed using an ellipsoid approximation. We present a mini-physiologically based pharmacokinetic/pharmacodynamic (mPBPK/PD) model that links drug concentrations in plasma and tumor to observed caspase activity and efficacy in an integrated manner, across multiple dose levels and schedules. The tumor is modeled as a pool of sensitive cells which can be triggered rapidly by AZD4320 to apoptose, from which point they transition gradually to death, reducing tumor volume. Cleaved caspase-3 is used as a marker of apoptosis, and modeled using a sigmoidal response function with steep slope parameter. In this way, we effectively capture the transient nature of the response, despite AZD4320's long residence in the tumor. Thrombocytopenia is described not as an effect on the megakaryocyte precursors, but as a linear concentration-dependent effect on circulating platelets. Feedback from the circulation to megakaryocytes drives increased production to fill the deficit. Parameters are well-estimated throughout. Together, these components provide a comprehensive means to investigate the effects of dose and schedule with a dual Bcl2/BCL-xL inhibitor. Citation Format: Francis D. Gibbons, Ammar Adam, Marie-Eve Beaudoin, Eric Gangl, Paul Secrist. Target engagement, thrombocytopenia, and efficacy induced by the dual Bcl2/xL inhibitor AZD4320 are quantitatively linked by a PK/PD model in leukemia xenografts [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3975.