Ovarian cancer's lethality is primarily a detection problem — roughly 75% of cases are diagnosed at Stage III or IV because the disease is asymptomatic until it has spread beyond the ovary, and no effective screening test exists for the general population. The PARP inhibitor approvals transformed the maintenance therapy landscape for BRCA-mutated and HRD-positive patients: olaparib maintenance after first-line chemotherapy extends progression-free survival by years, not months. But that benefit concentrates heavily in the ~20% of patients with germline BRCA1/2 mutations. The 2026 trial agenda is about what comes next — overcoming PARP inhibitor resistance, extending the benefit to HRD-negative patients, and validating mirvetuximab soravtansine's survival benefit across earlier lines of therapy.
This article is for informational purposes only and does not constitute medical advice. Clinical trial eligibility and availability vary. Always consult a qualified healthcare professional before making any medical decisions or considering participation in a clinical trial.
Summary
Ovarian cancer is diagnosed in approximately 20,000 women annually in the US with a 5-year overall survival around 49% — primarily because over 75% of cases present at advanced stage. PARP inhibitor maintenance has substantially extended PFS in BRCA-mutated and HRD-positive high-grade serous ovarian cancer (HGSOC). Mirvetuximab soravtansine (Elahere) became the first drug in years to show an overall survival benefit in platinum-resistant disease, with a 35% reduction in risk of death (OS HR 0.67) in the MIRASOL Phase 3 trial. In 2026, trials are targeting PARP resistance mechanisms, extending mirvetuximab to platinum-sensitive settings, and systematically evaluating the combination strategies needed to activate immunotherapy responses in a tumor type that has so far been largely resistant to checkpoint blockade.
ClinicalMetric Analysis
- BRCA reversion mutation detection via liquid biopsy during PARP inhibitor maintenance is clinically actionable — and earlier switch to ATR or WEE1 inhibitors before radiographic progression may improve subsequent response rates. Acquired resistance through BRCA reversion mutation restores functional HR repair and renders PARP inhibitors ineffective, but this happens at the molecular level before imaging shows progression. Serial ctDNA monitoring detecting reversion mutations during maintenance creates an opportunity to switch to mechanistically appropriate alternatives (ATR inhibitors for reversion-mediated resistance, ADCs for non-HR-mediated resistance) at a lower tumor burden. Centers with liquid biopsy capability should consider serial monitoring as a standard component of PARP inhibitor maintenance management rather than an investigational tool.
- Mirvetuximab soravtansine's OS benefit requires FRα testing that many community oncologists still aren't routinely ordering — patients diagnosed today may be unaware they should ask for it. MIRASOL enrolled patients with FRα-high expression (≥75% of cells at 2+ intensity by immunohistochemistry), and the OS HR of 0.67 applies to that specifically defined population. Patients with ovarian cancer who haven't had FRα testing on their archival tumor tissue may be unknowingly ineligible for the most effective approved second-line option in platinum-resistant disease. Every patient diagnosed with ovarian cancer should proactively ask about FRα testing at initial molecular profiling — even if they're currently being treated for platinum-sensitive disease, because the result will determine eligibility later.
- HRD testing beyond BRCA1/2 identifies an additional 20–25% of PARP inhibitor-eligible patients who are missed by germline BRCA testing alone. Genomic instability scores (Myriad myChoice CDx, FoundationOne CDx) detect HRD signatures — loss of heterozygosity, telomeric allelic imbalance, large-scale state transitions — in patients without BRCA mutations. PRIMA and PAOLA-1 established PARP inhibitor maintenance benefit in HRD-positive/BRCA-wild-type patients. Testing only germline BRCA without an HRD assay at diagnosis is an incomplete molecular workup for HGSOC. The test should be performed on tumor tissue (somatic HRD), not just blood (germline BRCA), to capture the full eligible population.
PARP Inhibitors: What the Data Shows and Where the Resistance Problem Sits
PARP inhibitors exploit synthetic lethality in homologous recombination-deficient (HRD) tumors. When BRCA1/2 mutations prevent HR-mediated DNA repair, cells become exquisitely dependent on PARP-mediated base excision repair. PARP inhibitors trap PARP proteins on damaged DNA, causing stalled replication forks and cell death specifically in HRD cells — a mechanism that leaves HR-proficient normal cells relatively unaffected.
The first-line maintenance trial data is robust. SOLO-1 (olaparib after first-line platinum in BRCA-mutant HGSOC) showed a 70% reduction in risk of disease progression or death (HR 0.30) at 2 years, with 7-year follow-up confirming sustained benefit — 46.8% of olaparib patients vs. 19.4% of placebo patients remained progression-free. PRIMA (niraparib after first-line platinum in HRD-positive population) showed HR 0.43 for PFS in the HRD-positive/BRCA-mutant subgroup. BRCA1/2-mutated patients derive the largest benefit, but HRD-positive/BRCA-wild-type patients also show meaningful improvement.
The resistance problem is where the 2026 trials concentrate. Acquired PARP inhibitor resistance typically develops through restoration of homologous recombination — BRCA reversion mutations that restore functional protein, loss of 53BP1 reducing end-joining and restoring HR, or drug efflux pump upregulation (MDR1/ABCB1). Several mechanisms are targetable:
- ATR inhibition combinations: Ceralasertib (AstraZeneca) and berzosertib (M6620) inhibit ATR kinase, which is required for replication fork stability when HR is impaired. PARP + ATR inhibitor combinations are in Phase 2 for PARPi-resistant HGSOC — the hypothesis being that ATR inhibition re-sensitizes cells that have restored partial HR. ORCA trial data is being analyzed.
- WEE1 inhibition: Adavosertib (AZD1775) inhibits WEE1 kinase, forcing premature mitotic entry in cells with DNA damage — synthetic lethality in HRD tumors. Phase 2 trials ongoing in PARPi-resistant settings.
- PARPi rechallenge: Whether patients who responded to first-line PARPi maintenance and progressed can benefit from a second PARPi course (potentially a different agent or after a treatment break) is being systematically evaluated. The OREO trial addresses rechallenge with olaparib vs. placebo after a treatment-free interval.
Mirvetuximab Soravtansine: What the MIRASOL Data Actually Means
Mirvetuximab soravtansine (Elahere, ImmunoGen/AbbVie) is an antibody-drug conjugate targeting folate receptor alpha (FRα), which is overexpressed in approximately 35–40% of high-grade serous ovarian cancers. The antibody (mirvetuximab) delivers a maytansinoid cytotoxin (DM4) directly to FRα-expressing cells with a cleavable linker that also allows bystander killing of adjacent FRα-negative cells that have taken up released drug from dying neighbors.
MIRASOL (NCT04209855), the Phase 3 randomized trial in platinum-resistant HGSOC with high FRα expression, compared mirvetuximab monotherapy vs. investigator's choice chemotherapy (paclitaxel, pegylated liposomal doxorubicin, or topotecan). Results: median PFS 5.62 vs. 3.98 months (HR 0.65, p<0.0001); median OS 16.46 vs. 12.75 months (HR 0.67, p=0.0046). This was the first randomized trial to demonstrate an overall survival benefit in platinum-resistant ovarian cancer in over a decade. FDA granted full approval in March 2024.
The 2026 trials expand mirvetuximab's use into platinum-sensitive settings where the benefit might be larger:
- GLORIOSA (NCT04672448): Mirvetuximab + bevacizumab as maintenance after response to platinum-based chemotherapy in FRα-high, platinum-sensitive relapsed disease. Phase 3, actively recruiting.
- PICCOLO (NCT05041257): Mirvetuximab + carboplatin in FRα-high platinum-sensitive relapsed HGSOC. Testing whether the ADC adds to standard platinum re-treatment.
The FRα testing requirement is important clinically. Not all ovarian cancer patients express FRα at the high level required for MIRASOL eligibility (≥75% of tumor cells at 2+ intensity). Patients and oncologists need to confirm FRα expression status — using the VENTANA FOLR1 (FOLR1-2.1) RxDx assay — before determining mirvetuximab eligibility. This is a companion diagnostic, not a standard pathology test.
Emerging ADC Targets: Broader Population Coverage
The MIRASOL success has accelerated ADC development across alternative targets that cover larger fractions of the ovarian cancer population:
Upifitamab rilsodotin (UpRi) — NaPi2b-Targeting ADC
NaPi2b (SLC34A2) is expressed in approximately 80% of ovarian cancers — a substantially broader population than FRα. The Phase 1/2 UPLIFT trial (NCT02606305) showed 35% ORR in heavily pre-treated patients (median 4 prior lines). Phase 3 UPLIFT-N is enrolling, comparing UpRi to platinum-based chemotherapy in NaPi2b-positive recurrent HGSOC. The broader target expression could make UpRi relevant to a much larger patient population than mirvetuximab.
Trastuzumab Deruxtecan (T-DXd) for HER2-Expressing Ovarian Cancer
HER2 is expressed (IHC 1+ or greater) in approximately 30% of ovarian cancers, though high-level expression (3+) is less common. The DESTINY-PanTumor02 basket trial (NCT04482309) included ovarian cancer patients with HER2 positivity (IHC 2+/3+), showing 45% ORR and 11.1-month median duration of response. These are meaningful numbers in heavily pre-treated disease, and dedicated Phase 2/3 ovarian cancer trials are in design. T-DXd's topoisomerase I inhibitor payload (deruxtecan) also confers a bystander killing effect that may be particularly relevant in tumors with heterogeneous HER2 expression.
Immunotherapy in Ovarian Cancer: Why It's Hard and What's Being Tried
High-grade serous ovarian cancer is immunologically "cold" — sparse tumor-infiltrating lymphocytes, low PD-L1 expression in most tumors, low tumor mutational burden (median 1.7 mutations/megabase), and a strongly immunosuppressive tumor microenvironment driven by TGF-beta, IDO1, and regulatory T-cell infiltration. Single-agent checkpoint inhibitors have shown response rates of 8–15% in unselected populations — not clinically meaningful as monotherapy.
But subsets do respond durably, and the 2026 trials are attempting to identify who responds and to expand that fraction through rational combination strategies:
- PARP + PD-1/PD-L1 combinations: PARP inhibitors increase mutational burden and upregulate PD-L1 — two mechanisms that might sensitize otherwise cold tumors to checkpoint blockade. DUO-O (olaparib + durvalumab + bevacizumab, NCT03737643) reported data in 2025 showing modest improvement in HRD-positive patients; the HRD-negative population showed less benefit.
- Neoantigen vaccine approaches: Personalized mRNA cancer vaccines encoding patient-specific neoantigens are entering Phase 2 in ovarian cancer in combination with checkpoint inhibitors, based on the KEYNOTE-942 melanoma data. The low mutational burden in HGSOC means fewer neoantigens to target, which is a real constraint — but the vaccine technology may compensate by choosing the most immunogenic subset.
- Anti-VEGF + checkpoint inhibitor: Bevacizumab targets the immunosuppressive tumor vasculature that limits T-cell infiltration. ATALANTE trial (bevacizumab + atezolizumab) showed benefit in a PD-L1-high subset, suggesting vascular normalization may facilitate immune infiltration in a biomarker-defined population.
Key Clinical and Biomarker Testing Requirements
- HRD status testing (Myriad myChoice CDx or Foundation Medicine equivalent) is required for PARP inhibitor maintenance eligibility and determines which patients derive the most benefit.
- FRα expression testing with the FDA-approved companion diagnostic is required before mirvetuximab therapy or mirvetuximab trial enrollment.
- Germline BRCA1/2 testing is recommended for all women with newly diagnosed ovarian cancer, with reflex somatic testing if germline testing is negative.
- HER2 testing (IHC and/or FISH) should be performed at recurrence, particularly in patients with platinum-resistant disease, to determine T-DXd or HER2-directed trial eligibility.
- Comprehensive genomic profiling (Foundation One CDx or equivalent) at recurrence ensures that rare but actionable alterations — NTRK fusions, MSI-H, NaPi2b overexpression — are not missed.