Preclinical toxicity evaluation of erythrocyte-encapsulated thymidine phosphorylase in BALB/c mice and beagle dogs: an enzyme-replacement therapy for mitochondrial neurogastrointestinal encephalomyopathy.

Erythrocyte-encapsulated thymidine phosphorylase (EE-TP) is currently under development as an enzyme replacement therapy for mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), an autosomal recessive disorder caused by a deficiency of thymidine phosphorylase. The rationale for the development of EE-TP is based on the pathologically elevated metabolites (thymidine and deoxyuridine) being able to freely diffuse across the erythrocyte membrane where the encapsulated enzyme catalyses their metabolism to the normal products. The systemic toxic potential of EE-TP was assessed when administered intermittently by iv bolus injection to BALB/c mice and Beagle dogs for 4 weeks. The studies consisted of one control group receiving sham-loaded erythrocytes twice weekly and two treated groups, one dosed once every 2 weeks and the other dosed twice per week. The administration of EE-TP to BALB/c mice resulted in thrombi/emboli in the lungs and spleen enlargement. These findings were also seen in the control group, and there was no relationship to the number of doses administered. In the dog, transient clinical signs were associated with EE-TP administration, suggestive of an immune-based reaction. Specific antithymidine phosphorylase antibodies were detected in two dogs and in a greater proportion of mice treated once every 2 weeks. Nonspecific antibodies were detected in all EE-TP-treated animals. In conclusion, these studies do not reveal serious toxicities that would preclude a clinical trial of EE-TP in patients with MNGIE, but caution should be taken for infusion-related reactions that may be related to the production of nonspecific antibodies or a cell-based immune response.

Synovial inflammation and hyperplasia induced by gliostatin/platelet-derived endothelial cell growth factor in rabbit knees.

Neovascularization, proliferation of synovial cells, and mononuclear cell influx and activation are characteristic events observed in synovial joints in the pathohistology of rheumatoid arthritis (RA). The objective of this study was to examine synovial inflammation in rabbit knees induced by intra-articular administration of human gliostatin/platelet-derived endothelial cell growth factor (GLS/PD-ECGF), which shares a high degree of chemical homology with thymidine phosphorylase (dThdPase) and is known to have angiogenic activity. Purified recombinant human gliostatin (rHuGLS) and its mutant protein, which was prepared by site-directed mutagenesis and which lacks dThdPase activity, were administered at various doses to rabbit knee joints. The effects of rHuGLS and the mutant were examined histologically. Intra-articular injection of rHuGLS resulted in the development of diffuse synovitis resembling RA. The mutant protein also brought about the same effect. These findings suggest that human GLS can cause RA-like synovitis in rabbit knee joints via a mechanism other than its dThdPase activity.