How is AKR1C3 (Aldo-Keto Reductase 1C3) targeted to resensitize resistant cells through AR-V7 (Androgen Receptor-V7) inhibition in prostate cancer?

Targeting AKR1C3 to Resensitize Resistant Cells Through AR-V7 Inhibition

Targeting AKR1C3 with pharmacologic inhibitors or genetic knockdown directly destabilizes and reduces AR-V7 protein expression, thereby resensitizing castration-resistant prostate cancer cells to antiandrogen therapies including enzalutamide, abiraterone, apalutamide, and darolutamide. 1

Mechanism of AKR1C3-Mediated AR-V7 Stabilization

The AKR1C3/AR-V7 axis represents a critical resistance mechanism in advanced prostate cancer:

• AKR1C3 physically interacts with AR-V7 protein, promoting its stabilization and preventing degradation, which maintains constitutive AR signaling independent of ligand binding 2, 1
• The AKR1C3/AR-V7 complex confers cross-resistance to all second-generation androgen receptor-targeted therapies, including enzalutamide, abiraterone, apalutamide, and darolutamide 1
• Chronic treatment with antiandrogens activates the steroid hormone biosynthesis pathway and increases AKR1C3 expression, creating a feed-forward resistance mechanism 1

Pharmacologic Approaches to Target AKR1C3

PROTAC Degraders

• The first-in-class AKR1C3 PROTAC degrader achieves potent AKR1C3 degradation with a DC50 of 52 nM in 22Rv1 prostate cancer cells 2
• Concomitant AR-V7 degradation occurs with a DC50 of 70 nM when AKR1C3 is degraded, demonstrating the direct dependency of AR-V7 stability on AKR1C3 presence 2
• This PROTAC also degrades AKR1C3 isoforms AKR1C1 and AKR1C2 to a lesser extent, providing broader steroidogenic enzyme inhibition 2

Small Molecule Inhibitors

PTUPB demonstrates superior effectiveness compared to indomethacin and celecoxib in suppressing AKR1C3 activity and CRPC cell growth 3:

• PTUPB inhibits AKR1C3 enzymatic activity while simultaneously blocking AR/AR-V7 signaling 3
• PTUPB synergizes with enzalutamide treatment in tumor suppression, providing benefits by blocking both AR-FL and AR-V7 signaling pathways 3
• Combination treatment with PTUPB and enzalutamide inhibits growth of castration-relapsed VCaP xenograft tumors and patient-derived xenograft organoids 3

LX1 represents a novel dual-targeting compound with enhanced therapeutic potential 4:

• LX1 binds to AKR1C3 active sites and inhibits enzymatic activity while simultaneously reducing AR/AR-V7 expression and target gene signaling 4
• LX1 inhibits conversion of androstenedione into testosterone in tumor-based ex vivo enzyme assays 4
• LX1 inhibits growth of cells resistant to enzalutamide, abiraterone, apalutamide, and darolutamide in vitro 4
• Synergistic effects occur when LX1 is combined with antiandrogens and taxanes, indicating potential for combination therapy in resistant disease 4
• LX1 treatment significantly decreases tumor volume, serum PSA levels, and intratumoral testosterone levels in CWR22Rv1 xenograft and LuCaP35CR patient-derived xenograft models 4

Genetic Knockdown Approaches

• Stable short hairpin RNA (shRNA) knockdown of AR-V7 in enzalutamide-resistant cells resensitizes cells to apalutamide and darolutamide treatment 1
• Targeting AKR1C3 with genetic knockdown resensitizes resistant cells to apalutamide and darolutamide through AR-V7 inhibition 1

Clinical Rationale for AKR1C3 Targeting

Steroidogenic Pathway Activation

• Reservoirs of DHEA-S remaining after castration and abiraterone treatment are converted to testosterone via AKR1C3 in amounts sufficient to stimulate prostate cancer cell growth 5
• AKR1C3 converts DHEA to 5-androstene-3β,17β-diol (5-Adiol) as the predominant precursor to testosterone, representing a major route of intratumoral androgen synthesis 5
• This mechanism operates in both primary (CWR22PC) and metastatic (DuCaP) prostate cancer cell lines, indicating broad applicability across disease stages 5

Cross-Resistance Prevention

• Enzalutamide- and abiraterone-resistant prostate cancer cells demonstrate cross-resistance to apalutamide and darolutamide through the AKR1C3/AR-V7 axis 1
• Chronic apalutamide treatment activates steroid hormone biosynthesis and increases AKR1C3 expression, conferring resistance to enzalutamide, abiraterone, and darolutamide 1

Therapeutic Strategy Algorithm

For patients with mCRPC progressing on novel hormone therapy 6:

1. Test for AR-V7 status in circulating tumor cells to identify patients most likely to benefit from AKR1C3 targeting, though AR-V7-positive patients are less likely to respond to abiraterone and enzalutamide 6

2. Consider AKR1C3 inhibition strategies in combination with antiandrogens rather than sequential antiandrogen therapy, as switching from one AR signaling inhibitor to another is rarely effective 6

3. Prioritize dual AKR1C3/AR-V7 targeting compounds (such as LX1) in combination with standard antiandrogens to simultaneously block intratumoral testosterone synthesis and AR variant signaling 4

4. Combine AKR1C3 inhibitors with taxane chemotherapy for synergistic effects, as AR-V7 status does not affect response to taxanes 6, 4

Critical Considerations

• AKR1C3 inhibition must be sustained to maintain AR-V7 destabilization, as the protective interaction between AKR1C3 and AR-V7 is continuous 2, 1
• Combination approaches targeting both AKR1C3 enzymatic activity and AR/AR-V7 protein expression provide superior outcomes compared to single-pathway inhibition 3, 4
• The prevalence of AR-V7 is low before treatment but increases with subsequent therapy lines, making AKR1C3 targeting increasingly relevant in later-line settings 6
• Patients with BRCA1/2 alterations should be considered for olaparib after novel hormonal agents rather than sequential antiandrogen therapy 6, 7