EPI-001 is the first inhibitor of the androgen receptor amino-terminal domain. The single stereoisomer of EPI-001, EPI-002, is a first-in-class drug that the USAN council assigned a new stem class "-aniten" and the generic name "ralaniten". This distinguishes the anitens novel molecular mechanism from anti androgens that bind the C-terminus ligand-binding domain and have the stem class "lutamide". EPI-001 and its stereoisomers and analogues were discovered by Dr. Marianne Sadar and Dr. Raymond Andersen who co-founded the pharmaceutical company ESSA Pharma Inc for the clinical development of anitens for the treatment of castration-resistant prostate cancer. EPI-001 is an antagonist of the androgen receptor that acts by binding covalently to the N-terminal domain of the AR and blocking protein-protein interactions required for transcriptional activity of the AR and its splice variants. This is different from all currently-used antiandrogens, which, conversely, bind to the C-terminal ligand-binding domain of the AR and competitively block binding and activation of the receptor by androgens. Due to its unique mechanism of action, EPI-001 type compounds may prove to be effective in the treatment of advanced prostate cancer resistant to conventional antiandrogens such as enzalutamide. EPI-001's successor, ralaniten acetate, a prodrug of ralaniten, one of the four stereoisomers of EPI-001, was under clinical investigation in a phase I study. EPI-506 was the first drug that directly binds to an intrinsically disordered region to be tested in humans and marks a leap in drug development from folded drug targets.
Pharmacology
Pharmacodynamics
EPI-001 is a mixture of four stereoisomers. EPI-001 binds to the activation function-1 region in the NTD of the AR, as opposed to other AR antagonists, which bind to the C-terminal LBD. A functional AF-1 is essential for the AR to have transcriptional activity. If AF-1 is deleted or mutated, the AR will still bind androgens, but will have no transcriptional activity. Importantly, if the AR lacks an LBD, the receptor will be nuclear and constitutively-active. Constitutively active splice variants of the AR that lack the C-terminal LBD are correlated to CRPC and poor survival. EPI-001 is an inhibitor of constitutively activesplice variant of ARs that lack the C-terminal LBD. Conventional antiandrogens do not inhibit constitutively-active variants of AR that have a truncated or deleted C-terminal LBD. In the absence of androgen, all known antiandrogens cause translocation of AR from the cytoplasm to the nucleus, whereas EPI-001 does not cause the AR to become nuclear. Binding of EPI-001 to the NTD of the AR blocks protein-protein interactions that are essential for its transcriptional activity. Specifically, EPI-001 blocks AR interactions with CREB-binding protein, RAP74, and between the NTD and C-terminal domain required for antiparallel dimer formation of AR. Unlike antiandrogens such as bicalutamide, EPI-001 does not cause the AR to bind to androgen response elements on the DNA of target genes. EPI-001 at extremely high concentrations of 50 to 200 uM has also been found to act as a selective PPARγ modulator, with both agonistic and antagonistic actions on the PPARγ. Via PPARγ activation, EPI-001 has been found to inhibit AR expression and activity in prostate cancer cells, indicating at least one AR-independent action by which EPI-001 exhibits antiandrogen properties in the prostate. EPI-001 inhibits AR-dependent proliferation of human prostate cancer cells while having no significant effects on cells that do not require the AR for growth and survival. EPI-001 has specificity to the AR and has excellent anti-tumor activity in vivo with xenografts of CRPC.
