PARP Inhibitors for Prostate Cancer: Olaparib, Rucaparib & More

What are PARP inhibitors?

PARP inhibitors (poly-ADP ribose polymerase inhibitors) are a class of targeted therapy drugs that block the PARP enzyme, which helps repair DNA damage in cells. They are particularly effective in cancers with existing DNA repair defects (HRR mutations).

PARP inhibitors are oral medications taken daily. They represent a major advance in precision medicine for prostate cancer.

How do PARP inhibitors work? – Synthetic lethality

PARP inhibitors exploit a concept called "synthetic lethality":

• Normal cells have multiple DNA repair pathways (PARP and HRR)
• Cancer cells with HRR mutations (BRCA, ATM, PALB2) rely on PARP for DNA repair
• PARP inhibitors block PARP, causing DNA damage to accumulate
• HRR-deficient cancer cells cannot repair this damage and die
• Normal cells (with intact HRR) are unaffected

Which mutations predict response? – BRCA, ATM, PALB2, HRR genes

HRR (homologous recombination repair) gene mutations predict response to PARP inhibitors:

• BRCA2 (most common in prostate cancer): Strongest predictor of response (50-80% response rate)
• BRCA1: Also predicts response (less common in prostate cancer)
• ATM: Moderate predictor (30-40% response rate)
• PALB2, CHEK2, RAD51, BRIP1, FANCA: Other HRR genes (variable response)

Prevalence of HRR mutations in prostate cancer:

• Metastatic prostate cancer: 20-30% have HRR mutations
• BRCA2: 5-10% of metastatic cases
• ATM: 5-10%
• Other HRR genes: 5-10%

Olaparib (Lynparza) – first FDA-approved

Olaparib was the first PARP inhibitor approved for prostate cancer (FDA 2020).

Approval:

• Metastatic castration-resistant prostate cancer (mCRPC)
• With HRR gene mutation (BRCA1/2, ATM, PALB2, etc.)
• Progressed after enzalutamide or abiraterone

Dosing:

• 300 mg twice daily (oral tablets)
• Continuous dosing (no breaks)

Key trial (PROfound):

• Olaparib improved median radiographic progression-free survival (rPFS): 7.4 vs. 3.6 months
• In BRCA-mutated subgroup: rPFS 9.8 vs. 3.0 months
• Overall survival benefit: 19.1 vs. 14.7 months

Rucaparib (Rubraca) – second approved option

Rucaparib was approved for prostate cancer in 2020 (FDA).

Approval:

• Metastatic castration-resistant prostate cancer (mCRPC)
• With BRCA1/2 mutation (not other HRR genes)
• Progressed after enzalutamide or abiraterone

Dosing:

• 600 mg twice daily (oral tablets)

Key trial (TRITON2):

• Objective response rate: 44% in BRCA-mutated patients
• Median radiographic progression-free survival: 9.0 months

Niraparib and talazoparib – emerging options

Other PARP inhibitors are being studied for prostate cancer:

Niraparib (Zejula):

• Approved for ovarian cancer, not yet for prostate cancer
• Phase 3 trial (MAGNITUDE) showed benefit in HRR-mutated mCRPC
• FDA approval pending

Talazoparib (Talzenna):

• Approved for breast cancer, not yet for prostate cancer
• Phase 3 trial (TALAPRO-2) showed benefit in mCRPC (with enzalutamide)
• FDA approval pending

Who should be tested? – All men with metastatic prostate cancer

Genetic testing for HRR mutations is now standard of care:

• All men with metastatic prostate cancer (NCCN guidelines, 2025)
• Testing can be done on tumor tissue (somatic testing) or blood (germline testing)
• Recommend both tumor and germline testing

Genes to test:

• BRCA1, BRCA2, ATM, PALB2, CHEK2, RAD51, BRIP1, FANCA, FANCL, FANCM, HOXB13 (minimum panel)

Why test?

• Identifies patients who may benefit from PARP inhibitors
• Identifies hereditary cancer syndromes (family implications)
• Guides prognosis (BRCA2 mutations indicate aggressive disease)

Effectiveness – response rates and progression-free survival

PARP inhibitors are highly effective in BRCA-mutated prostate cancer:

Response rates (by mutation):

• BRCA2: 50-80% PSA response, 40-60% objective response
• BRCA1: 40-60% PSA response
• ATM: 20-40% PSA response
• Other HRR genes: 10-30% PSA response

Progression-free survival (olaparib, PROfound trial):

• BRCA-mutated: 9.8 months (vs. 3.0 months for control)
• All HRR-mutated: 7.4 months (vs. 3.6 months for control)

Duration of response:

• Median duration: 6-9 months for BRCA mutations
• Resistance eventually develops (usually within 1-2 years)

Side effects – anaemia, fatigue, nausea

PARP inhibitors are generally well-tolerated but have predictable side effects:

Common side effects (20-40%):

• Anaemia (low red blood cells): Most common dose-limiting toxicity
• Fatigue: 30-40% – manageable with rest and activity modification
• Nausea: 20-30% – usually mild, treat with anti-emetics
• Decreased appetite: 10-20%

Less common (5-10%):

• Thrombocytopenia (low platelets): Increased bleeding risk
• Neutropenia (low white blood cells): Increased infection risk
• Fatigue: Can be dose-limiting

Serious but rare (<5%):

• Myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML): Rare (<1% after 2+ years of treatment)

Interactive FAQ – PARP inhibitors for prostate cancer

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Disclaimer: This information is for educational purposes. PARP inhibitor treatment should be guided by genetic testing and medical oncology. Consult a specialist at Vivekananda Hospital.