Overcoming CD28-Mediated Multi-Drug Resistance With a Novel PIM2 Inhibitor

Drug resistance in multiple myeloma (MM) is the major cause of treatment failure and is significantly mediated by pro-survival interactions with bone marrow microenvironment. A key myeloma receptor involved in this interaction is CD28, which has been largely characterized as the prototypic T cell costimulatory receptor. However, CD28 expression on myeloma cells is significantly correlated with disease progression, worse prognosis, and is significantly higher in the poor prognosis t(14;16) MAF subgroup. We now report that CD28 signaling mediates significant drug resistance and protects MM against death from multiple chemotherapeutics with different mechanisms of action – including dexamethasone, arsenic trioxide, melphalan or bortezomib. Inhibition of specific signaling (PI3K or Akt) or targets (Foxo3A or Bim) downstream of CD28 activation abrogates this protection. Unexpectedly, we found evidence that the PIM2 kinase (which is largely uncharacterized in MM but is also significantly overexpressed in the MAF subgroup) may be a previously unreported component of the CD28 pro-MM survival pathway. The novel small molecule PIM2 inhibitor JP_11646 (IC₅₀ 0.5 nM) abrogates CD28-mediated protection against apoptosis in MM cell lines in vitro, which we have not previously seen for any chemotherapeutic tested. In addition, blockade of CD28 activation sensitized MM cells significantly to JP_11646-induced death. Altogether, these data suggested that PIM2 inhibitors can overcome a major mechanism of multi drug resistance in MM.

Jasco’s novel and selective pan-PIM inhibitor (JP_11646) has demonstrated biochemical IC50s of 24, 0.5 and 1 nM for PIM1, PIM2 and PIM3 respectively. The PIM mechanism of action has been confirmed through cell based transphosphorylation assays, where JP_11646 decreased PIM dependent phoshphorylation of the proapoptotic protein BAD at nM levels. JP_11646 increases apoptosis and decreases cell viability in multiple myeloma cell lines with the MAF translocations (<100 nM). JP_11646 is orally bioavailable and has demonstrated in vivo efficacy, inhibiting tumor growth by >80% in a MM1.S tumor xenograft study. These data provide solid rationale for further development of JP_11646 as a targeted therapy in MM, and specifically for patients exhibiting the MAF translocation.