Evaluation of intravitreal topotecan dose levels, toxicity and efficacy for retinoblastoma vitreous seeds: a preclinical and clinical study

Abstract

BackgroundCurrent melphalan-based intravitreal regimens for retinoblastoma (RB) vitreous seeds cause retinal toxicity. We assessed the efficacy and toxicity of topotecan monotherapy compared with melphalan in our rabbit model and patient cohort.MethodsRabbit experiments: empiric pharmacokinetics were determined following topotecan injection. For topotecan (15 μg or 30 µg), melphalan (12.5 µg) or saline, toxicity was evaluated by serial electroretinography (ERG) and histopathology, and efficacy against vitreous seed xenografts was measured by tumour cell reduction and apoptosis induction. Patients: retrospective cohort study of 235 patients receiving 990 intravitreal injections of topotecan or melphalan.ResultsIntravitreal topotecan 30 µg (equals 60 µg in humans) achieved the IC90 across the rabbit vitreous. Three weekly topotecan injections (either 15 µg or 30 µg) caused no retinal toxicity in rabbits, whereas melphalan 12.5 µg (equals 25 µg in humans) reduced ERG amplitudes 42%–79%. Intravitreal topotecan 15 µg was equally effective to melphalan to treat WERI-Rb1 cell xenografts in rabbits (96% reduction for topotecan vs saline (p=0.004), 88% reduction for melphalan vs saline (p=0.004), topotecan vs melphalan, p=0.15). In our clinical study, patients received 881 monotherapy injections (48 topotecan, 833 melphalan). Patients receiving 20 µg or 30 µg topotecan demonstrated no significant ERG reductions; melphalan caused ERG reductions of 7.6 μV for every injection of 25 µg (p=0.03) or 30 µg (p<0.001). Most patients treated with intravitreal topotecan also received intravitreal melphalan at some point during their treatment course. Among those eyes treated exclusively with topotecan monotherapy, all eyes were salvaged.ConclusionsTaken together, these experiments suggest that intravitreal topotecan monotherapy for the treatment of RB vitreous seeds is non-toxic and effective.

Intravitreal HDAC Inhibitor Belinostat Effectively Eradicates Vitreous Seeds Without Retinal Toxicity In Vivo in a Rabbit Retinoblastoma Model

“…Second, our previous work with other drugs had demonstrated that intravitreal and intra-arterial injections of various other drugs leads to high but transient vitreous concentrations, with relatively rapid efflux. 24 , 25 Thus, following intravitreal injection, the vitreous seeds would only be exposed to the initial high concentration very briefly. We therefore determined the vitreous half-life to ensure that the dose calculated for the initial injection would allow a concentration (on the opposite side of the vitreous) above the IC 90 for a length of time equivalent to at least 5 half-lives.…”

“…Human WERI-Rb1 retinoblastoma cells were grown in RPMI with 10% fetal bovine serum under normal growth conditions, as reported previously. 24 , 33 Five thousand cells were plated in 96-well plates and exposed to belinostat at various concentrations for 4 hours (equivalent to 5 half-lives as determined through the above-described pharmacokinetic experiments). The belinostat-containing media were then removed and replaced with fresh media.…”

“…Additional cohorts of rabbits received equivalent volumes of saline injections as a control, or received melphalan 12.5 µg as a comparator of the toxicity associated with current standard-of-care treatment. Because the rabbit vitreous volume is half the size of a human child's vitreous volume, experiments with intravitreal injections in rabbits generally have used a dose equal to half that used in patients, 24 , 33 – 35 and we have maintained that accepted conversion factor in these experiments. In our study, 12.5 µg represents the rabbit vitreous volume-adjustment of the clinical dose in humans (25 µg).…”

“…Toxicity was defined prospectively as described previously. 14 , 24 , 33 , 35 Briefly, toxicity was deemed significant for a given dose in a rabbit group if there was a 25% reduction in average ERG amplitude or a 25% prolongation of average implicit time for a given parameter when comparing the post-treatment values with the pre-treatment values, and the difference was statistically significant. For assessment of toxicity in individual rabbits, toxicity was defined as a 25% reduction in ERG amplitude or a 25% prolongation of implicit time for a given parameter when comparing the post-treatment values with the pre-treatment values for that rabbit.…”

“…However, these alternative drugs used intravitreally have generally either repurposed already-used intravenous agents (e.g., carboplatin, etoposide) 20 , 21 or simply repurposed already-used intra-arterial agents (e.g., topotecan). 22 – 24 These have generally first been tried clinically, without first being tested in preclinical studies to demonstrate efficacy or lack of toxicity or to inform dosing decisions.…”