Protamine zinc insulin (PZI), a long-acting insulin preparation in which protamine and zinc are complexed with insulin to delay absorption, has good glycemic control in 85% to 90% of diabetic cats. Proper release of PZI depends on consistent formulation of its 3 critical components: protamine, zinc, and insulin. Originally, PZI was of bovine and porcine origin, but an FDA-approved human recombinant PZI product now available reportedly has comparable efficacy. Alternative PZI products are available through veterinary compounding pharmacies.
This study compared the characteristics of a commercially manufactured PZI product with those of PZI products from 12 compounding pharmacies. A total of 112 vials (16 commercial samples, 8 samples from each compounding pharmacy) were purchased over 8 months. Products were analyzed for endotoxin concentration, crystal size, insulin concentration in the supernatant, pH, total insulin and zinc concentrations, and species of insulin origin. The 16 commercially manufactured samples met US Pharmacopeia (USP) specifications. Of the compounded products, only 1 of the 96 vials tested met USP specifications; furthermore, 54% did not meet specifications for zinc concentration and 38% had total insulin concentrations <90% of the labeled concentration.
Commentary
PZI and glargine insulin are currently the most frequently recommended insulin products for treatment of diabetic cats. This study arms veterinarians with evidence against the use of compounded PZI. The less costly option may be attractive to clients, but the variation in insulin concentrations, even among vials from the same source, can lead to unregulated diabetes and/or hypoglycemia. The extra cost associated with identifying why a cat is no longer regulated can outweigh the initial savings of using compounded PZI. Although the commercial PZI product evaluated in this study is no longer manufactured, another FDA-approved product manufactured with the same current good manufacturing practices and quality-control standards is available.—Patty Lathan, VMD, DACVIM
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