An Animal Model Suitable for Predicting In Vivo Behavior of Platelets in Humans: A Tool for New Product Development.

Testing platelet in vivo activity in an animal model remains an attractive approach for the pre-clinical evaluation of platelets that have been treated or manipulated by a new technology or product. Mirasol PRT™ is a novel pathogen reduction technology based on riboflavin photochemistry for the safety of blood component transfusion. In this study, rabbit platelets were tested for viability after exposure to 2, 3 and 5 J/cm² of UV light in the presence of 50 μM riboflavin in a polypropylene bag (Sangewald). In order to minimize the potential variations caused by different isotope labeling and by individual rabbit differences, a pool of platelets was split into two for the treatment and control groups. Eight to ten rabbits were used in each group as recipients for each dose of light. Half of the rabbits in the group received concurrent injections of ¹¹¹In-labeled PRT treated platelets and ⁵¹Cr-labeled control platelets. The other half received ⁵¹Cr-labeled PRT-platelets and ¹¹¹In-labeled control platelets. The recovery and survival times were determined post-infusion for both treated and control platelets using the multiple hit model (γ-function) and data from the various groups were pooled and analyzed. The platelets treated with 2, 3 and 5 J/cm² retained recoveries of 63.2% (SD=17, n=9), 55.3% (SD=10.8, n=10) and 44.0% (SD=12.8, n=8) of control platelets, respectively. The survival times of the platelets treated with 2, 3 and 5 J/cm² were 59.5% (SD=15.7, n=9), 60.1% (SD=19.5, n=10) and 55.1% (SD=18.1, n=8) of control, respectively. Both values of recovery and survival times were correlated with UV light dose with correlation coefficients of 0.90 (r²) and 0.73 (r²) respectively. In a subsequent single site human clinical study, human platelets were treated with UV light at a dose of 3 J/cm² in a Sangewald bag, radiolabelled with Indium at day 5 of storage and infused into autologous subjects. Recovery and survival values for these subjects were obtained. At a dose of 3 J/cm², recovery and survival activities retained 54.9% (SD=3.1, n=4) and 71.7% (SD=21.5) of control platelets, respectively. Good agreement with observations made in the rabbit model was thus obtained. Based on this work, a light dose was selected for standard treatment of human platelets using the Mirasol PRT system. In conclusion, this kinetic study on UV light dose in an animal model was predictive of human platelet in vivo activity, making it a potentially valuable model for estimating human clinical trial outcomes for new platelet storage and treatment methodologies.