Establishing Clinical Trial Endpoints in Selecting Patients for RPGR Retinal Gene Therapy.

Purpose: Clinical trials for X-linked retinitis pigmentosa (RP) often assess retinal structure using optical coherence tomography (OCT) and function using microperimetry to evaluate initial eligibility and endpoints. Therefore, we seek to determine which parameters might be most sensitive in screening new patients for enrollment. Methods: Thirty-one patients (62 eyes) with confirmed retinitis pigmentosa GTPase regulator (RPGR) mutations attending Oxford Eye Hospital were included in this retrospective analysis. Outer retinal structure was investigated by measuring the remaining ellipsoid zone (EZ) and external limiting membrane (ELM) on OCT. Visual function was evaluated by using 10-2 microperimetry mean sensitivity. Results: The median age of patients with RPGR was 31 years (interquartile range [IQR] = 22-39 years). For the right and left eyes, respectively, the median EZ length through the foveal section was 921 µm (IQR = 607-1570) and 865 µm (IQR = 508-1442) and median ELM length was 2056 µm (IQR = 1336-2764) and 1860 µm (IQR = 1152-2680). Similarly, the median microperimetry sensitivity (MS) was 2.0 decibel (dB; IQR = 0.4-5.4) and 1.1 dB (IQR = 0.1-5.4). Linear mixed model regression analysis showed that EZ was significantly positively correlated to ELM (P < 0.001, R² = 0.931). EZ and ELM were significantly correlated with the microperimetry sensitivity with exponential relationship (P < 0.001, R² = 0.71 and 0.72, respectively). Using the exponential equation of regression line, EZ below approximately 600 µm (RE = 637 µm, 95% confidence interval [CI] = 397-877, LE = 586 µm, 95% CI = 356-817) results in microperimetry sensitivity of approximately 0 dB. There was high degree of inter-eye symmetry for progression of EZ, ELM, and microperimetry sensitivity. Age was significantly correlated with the analyzed parameters (P < 0.001), although with low R² for each of them. Discussion: EZ may comprise a surrogate biomarker for prediction of visual function in X-linked RP caused by mutations in RPGR and, in turn, identification of appropriate patients for enrollment in clinical trials. As expected, age plays a role in predicting potential biomarkers and visual function, however, it should not be used to preclude patients for gene therapy due to the poor correlation and heterogeneity of disease onset. Translational Relevance: Biomarkers of visual function in RPGR-associated RP may lead to identification of appropriate patients for enrollment in clinical trials.