Decreasing the intratumoral androgen biosynthesis by using an inhibitor of 17β-hydroxysteroid dehydrogenase type 3 (17β-HSD3) is a strategy to treat prostate cancer. The androsterone (ADT) derivative 1 (RM-532-105) has shown strong inhibitory activity on 17β-HSD3, but needs to be improved. Herein, we describe the chemical synthesis and characterization of two series of analogues to address the impact of A- and D-ring modifications on 17β-HSD3 inhibitory activity, androgenic effect, and metabolic stability. Structure-activity relationships were generated by adding different groups at C16/C17 (D-ring diversification) or replacing the ADT backbone by a nor-androstane or an estrane backbone (A-ring diversification). D-ring derivatives were less potent inhibitors than lead compound 1, whereas steroidal backbone (A-ring) change led to identifying promising novel estrane derivatives. This culminated with potent 17β-HSD3 inhibitors 23, 27, 31, and 33 (IC50 = 0.10, 0.02, 0.13, and 0.17 μM, respectively), which did not stimulate LAPC-4 cell proliferation and displayed higher plasma concentration in mice than lead compound 1.