PV701 warrants further study as a novel therapeutic agent for cancer patients.

Purpose: Previous phase 1 trials of i.v.-administered PV701 have shown this virus to be welltolerated with toxicity primarily associated with the first dose. Our hypothesis, based on preclinical evidence, was that patient tolerability could be improved by slowing the i.v. infusion rate, and that this approach would allow for the safe administration of higher doses. Additionally, this phase 1trial was the first to measure PV701clearance. Experimental Design: For the first dose, a 3-h infusion was used compared with the 10-and 30-min infusions administered in the two previous trials. Subsequent doses were infused over 1h. Six doses were given per 3-week cycle. Escalation of the first dose was done separately from the escalation of doses 2 to 6. Viral clearance was determined using whole blood reverse transcription-PCR. Results: Eighteen patients with advanced chemorefractory cancer were enrolled. The first dose was safely escalated to 24 Â 10 9 plaque-forming units/m 2 and doses 2 to 6 were safely escalated to 120 Â 10 9 plaque-forming units/m 2 . Tolerability was improved compared with the rapid bolus dosing used previously with the elimination of severe flu-like symptoms. Furthermore, infusion reactions were markedly decreased in this trial compared with previous PV701trials.The presence of neutralizing antibodies did not significantly affect PV701clearance. Four major and two minor tumor responses were observed. Conclusions: Using slow infusion, patient tolerability was improved, while the first dose was safely escalated relative to two previous PV701 trials. Based on improved tolerability and encouraging signs of activity, this slow infusion regimen was selected for further PV701 clinical development.