EphB4-ephrin-B2 are targets in castration resistant prostate cancer.

Background: PI3K pathway activation is a common and early event in prostate cancer, from loss of function mutations in PTEN, or activating mutations in PIK3Ca or AKT leading to constitutive activation, induction of growth factor-receptors kinase EphB4 and its ligand ephrin-B2. We hypothesized that induction of EphB4 is an early event required for tumor initiation. Secondly, we hypothesized that EphB4 remains relevant when prostate cancer becomes androgen independent.

Methods: Genetic mouse model of conditional PTEN deletion in prostate epithelium induces tumor in all mice. We tested this model against EPHB4 wild type and deleted in prostate epithelium. This allowed us to test its role in tumor initiation. We also tested an orthogonal approach by using decoy soluble EphB4 to block bidirectional signaling resulting from EphB4-ephrin-B2 interaction. Role of EphB4-ephrin-B2 in androgen deprived mice was tested for role in refractory cancer model.

Results: PTEN deletion induces EphB4 and ephrin-B2 in prostate cancer which was substantially reduced when EPHB4 is deleted in the same prostate epithelial cells. sEphB4-alb fusion protein with improved pharmacokinetics similarly inhibited tumor formation, thus establishing the role in tumor initiation. sEphB4-alb retained the efficacy in castration resistant androgen independent prostate cancer. We have thus observed that induction of EphB4 is required for the initiation of prostate cancer in PTEN null mouse and that signaling downstream from EphB4 is required in androgen deprivation and thus castration resistant prostate cancer. Pharmacological inhibition of EphB4 pathway reproduced the results. Targeting EphB4 should be tested in prostate cancer especially those resistant to androgen deprivation therapy.

Conclusions: EphB4 and ephrin-B2 receptor ligand pair is induced in PTEN null prostate cancer, which significantly contributes to the tumor initiation. Secondly, EphB4-ephrin-B2 pathway continue to promote tumor progression even in androgen deprivation and thus hormone refractory tumor. EphB4-ephrin-B2 may be candidates for precision medicine with biomarker-based patient selection with and without concurrent standard of care.