Ocular and systemic toxicity of high-dose intravitreal topotecan in rabbits: Implications for retinoblastoma treatment

Although many more eyes of children with retinoblastoma are salvaged now compared to just 10 years ago, the control of vitreous seeding remains a challenge. The introduction of intravitreal injection of melphalan has enabled more eyes to be salvaged safely but with definite retinal toxicity. Intensive treatment with high-dose intravitreal topotecan may be a strategy to control tumor burden because of its cell cycle-dependent cytotoxicity and the proven safety in humans. Therefore, we evaluated the ocular and systemic safety of repeated high-dose intravitreal injections of topotecan in rabbits. Systemic and ocular toxicity was assessed in non-tumor-bearing rabbits after four weekly injections of three doses of topotecan (10μg, 25μg, and 50μg) or vehicle alone. Animals were evaluated weekly for general and ophthalmic clinical status. One week after the last injection, vitreous and plasma samples were collected for drug quantification and the enucleated eyes were subjected to histological assessment. Weight, hair loss, or changes in hematologic values were absent during the study period across all animal groups. Eyes injected with all topotecan doses or vehicle showed no signs of anterior segment inflammation, clinical or histologic evidence of damage to the retina, and ERG parameters remained unaltered throughout the study. Vitreous and plasma topotecan lactone concentrations were undetectable. Four weekly intravitreal injections of topotecan up to 50μg in the animal model or a 100μg human equivalent dose were not toxic for the rabbit eye. High doses of topotecan may show promising translation to the clinic for the management of difficult-to-treat retinoblastoma vitreous seeds.

Context-sensitive recovery of neuromuscular function from vecuronium in dogs: Effects of dose and dosing protocol

Recovery of neuromuscular function is a gradual phenomenon whereby function progresses from absent to normal. The speed of spontaneous recovery can be used to predict the time when neuromuscular function is expected to be restored. However, the speed of recovery might be affected by the dose of the neuromuscular blocker administered, and by the dosing regimen of that dose. The effects of both factors on the speed of spontaneous recovery from vecuronium were evaluated. Seven dogs were anesthetized three times and the train-of-four (TOF) ratio was measured with acceleromyography. Vecuronium was administered at 0.1 mg/kg, 0.2 mg/kg, or 0.1 mg/kg followed by two doses of 0.05 mg/kg was administered each time. In the divided-dose treatment group, aliquots were administered on return of the first twitch (T1) of the TOF from the previous dose. The duration of surgical block, from injection to return of T1, was longest for the divided-dose protocol, intermediate for 0.2 mg/kg single bolus, and shortest for 0.1 mg/kg (P <  0.0001). The recovery period, from return of T1 to a TOF ratio ≥0.9, was longer for 0.2 mg/kg administered as a single bolus than for the other two groups (P = 0.007). Doubling the dose of a single bolus of vecuronium extended the time of surgical block and prolonged the duration of the recovery period. However, dividing that dose into smaller aliquots extended the period of surgical block while shortening the recovery period. Hence, the spontaneous reappearance of T1 should not be used in isolation to predict the time to complete recovery of neuromuscular function.