Limited evidence exists to support CA-125 as a valid surrogate biomarker for progression in patients with ovarian cancer on maintenance PARP inhibitor (PARPi) therapy. We aimed to assess the concordance between CA-125 and Response Evaluation Criteria in Solid Tumours (RECIST) criteria for progression in patients with BRCA mutations on maintenance PARPi or placebo.
We extracted data on progression as defined by Gynecologic Cancer InterGroup CA-125, investigator- and independent central-assessed RECIST from the SOLO2/ENGOT-ov21(NCT01874353) trial. We excluded those with progression other than by RECIST, progression on date of randomisation, and no repeat CA-125 beyond baseline. We evaluated the concordance between CA-125 progression and RECIST progression, and assessed the negative (NPV) and positive predictive value (PPV).
Of 295 randomised patients, 275 (184 olaparib, 91 placebo) were included. 171 patients had investigator-assessed RECIST progression. Of 80 patients with CA-125 progression, 77 had concordant RECIST progression (PPV 96%, 95% confidence interval 90-99%). Of 195 patients without CA-125 progression, 94 had RECIST progression (NPV 52%, 45-59%). Within treatment arms, PPV was similar (olaparib: 95% [84-99%], placebo: 97% [87-100%]) but NPV was lower in patients on placebo (olaparib: 60% [52-68%], placebo: 30% [20-44%]). Of 94 patients with RECIST but without CA-125 progression, 64 (68%) had CA-125 that remained within normal range. We observed similar findings using independent-assessed RECIST.
Almost half the patients without CA-125 progression had RECIST progression, and most of these had CA-125 within the normal range. Regular computed tomography imaging should be considered as part of surveillance in patients treated with or without maintenance olaparib rather than relying on CA-125 alone.

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