Effect of ruboxistaurin (RBX) On visual acuity decline over a 6-year period with cessation and reinstitution of therapy: results of an open-label extension of the Protein Kinase C Diabetic Retinopathy Study 2 (PKC-DRS2)

Abstract

Purpose: The PKC-DRS2 was a Phase 3, randomized, double-masked, placebo (PBO)-controlled, 3-year study of the effect of 32 mg/day of ruboxistaurin (RBX), an orally administered protein kinase C inhibitor, on vision loss in patients with moderate to severe nonproliferative diabetic retinopathy. Ruboxistaurin reduced the occurrence of sustained moderate visual loss (SMVL; ≥15-letter decline in visual acuity sustained for the last 6 months of study participation) from 9.1% in the PBO group (N = 340) to 5.5% in the RBX group (N = 345, P = 0.034). This study evaluates the primary end point of SMVL in a 2-year open-label extension (OLE) of the PKC-DRS2, which began a median of 466 days (range, 263-1,296 days) after the conclusion of PKC-DRS2.

Methods: Visual acuity was measured by certified examiners using the Early Treatment Diabetic Retinopathy Study chart.

Results: Of the 514 patients who completed PKC-DRS2, 366 did so in the 32 study centers participating in the OLE, and of these, 203 (55%) enrolled in the OLE for treatment with 32 mg/day of RBX for 2 years. Of the 203 enrolled in the OLE, 100 had previously been treated with PBO (prior PBO subgroup) and 103 had been treated with RBX (prior RBX subgroup). PKC-DRS2 baseline patient and ocular characteristics were well matched between these two subgroups and were similar to the PKC-DRS2 patient population as a whole. Using the PKC-DRS2 baseline as the starting point, SMVL occurred in 6% of the prior PBO subgroup during the PKC-DRS2, increasing to 26% by the end of the OLE. However, in the prior RBX subgroup, SMVL occurred in only 4% and 8% during the PKC-DRS2 and by the end of the OLE, respectively (P < 0.001 for difference at the end of the OLE). In the prior PBO subgroup, mean visual acuity declined from 79.6 letters at PKC-DRS2 baseline to 73.1 letters at OLE end point (-6.5 letters). In the prior RBX subgroup, this loss was 2.7 letters (79.8 to 77.1) over the same period (P = 0.02).

Conclusion: Over a 6-year study period incorporating 3 years of a rigorously placebo-controlled trial, approximately 1 year off treatment and 2-year OLE where all groups received therapy, those patients with greatest RBX exposure (∼5 years) experienced less SMVL compared with those in the original PBO group (∼2-year RBX exposure).