In vivo distribution of radioactivity in mice after injection of biodegradable polymer microspheres containing C-14-labeled tetanus toxoid

Radiolabeled tetanus toroid (TT) was prepared by detoxifying chromatographically purified tetanus toxin with C-14-labeled formaldehyde. C-14-TT was encapsulated inside poly(D,L-lactide-co-glycolide, 50/50) microspheres (MS) of varying average size (similar to 10 mu m and similar to 50 mu m). Balb/c mice were injected subcutaneously with 5 Lf (similar to 15 mu g) of C-14-TT, encapsulated in MS, mixed with blank MS without encapsulated antigen, as soluble antigen or adsorbed onto aluminum phosphate (AlPO4) and radioactivity was monitored at the site of injection, draining lymph nodes, blood, liver, spleen, and kidneys at various intervals. At one day, similar to 95% and 90% radioactivity disappeared from site of injection for soluble TT or blank MS mixed TT and AlPO4 adsorbed TT, respectively, whereas similar to 55% and 70% radioactivity disappeared from site of injection for MS of average size similar to 50 mu m and similar to 10 mu m, respectively. But 7 days, 99% of radioactivity disappeared from site of injection far soluble TT or blank MS mixed TT, whereas 2-3% radioactivity persisted at the site of injection for AlPO4 adsorbed TT for 4 weeks. In contrast, similar to 20% radioactivity stayed at the site of injection for MS injected mice up to 4 weeks. At all time points, large MS (similar to 50 mu m) showed more radioactivity at the site of injection than small MS (similar to 10 mu m). Other organs showing radioactivity were draining lymph nodes and kidneys. Small MS with encapsulated TT showed highest level of radioactivity in lymph Modes at 4 h. In kidneys, soluble and AlPO4 adsorbed TT showed a peak of radioactivity at 4 h whereas TT encapsulated in MS showed a peak of radioactivity at 7 days. These results indicate that AlPO4 did not act as a depot for TT at the site of injection, bur TT encapsulated in MS did form a depot for approximately 1 month. Copyright (C) 1996 Elsevier Science Ltd.