Calcium sensitive feedback release of parathyroid hormone as a potential treatment of hypoparathyroidism

Abstract

Introduction. The most common cause of hypoparathyroidism is iatrogenic injury to the parathyroid glands during thyroid resection. To avoid adverse side effects of long-term calcium and vitamin D therapy, we are investigating ways to maintain calcium homeostasis through the controlled release of parathyroid hormone (PTH) from poly(lactide-co-glycolide) (PLGA) microspheres. An ideal delivery vehicle for this purpose would be biodegradable and sensitive to calcium concentrations. Since calmodulin undergoes a conformational change when bound with calcium, we hypothesized that calmodulin, co-encapsulated with PTH, may alter the PTH release kinetics from biodegradable PLGA microspheres. Methods. Using double emulsion solvent evaporation technique, PLGA microspheres loaded with 0.5 mg of PTH and 50 mg of calmodulin were made. In vitro release profiles for the microspheres were determined by incubating them in a Tris-HCl buffer at pH 7.5, 37°C and varying calcium chloride (CaCl 2) concentration. The effluent was serially sampled and PTH concentration measured. First, we compared release of PTH with (2mM) CaCl 2 and without CaCl 2 in the buffer. Second, we constructed the release profile with CaCl 2 at concentrations of 1 and 2 mM. Next, bioactivity of the PTH released from these microspheres was evaluated in vivo by injecting 25 mg subcutaneously in a rat and obtaining serial serum levels. Results. (1) In experiments in which effect of calcium chloride was compared to no CaCl 2, after 3 h, 2 mM Ca 2+ buffer had reduced the release of PTH by 20%. In the experiments with buffer containing 1 and 2 mM of CaCl 2, the microspheres released 110 ng less of PTH at a CaCl 2 concentration of 2 mM than at 1 mM. When the buffers were reversed, rate of release of PTH from the microspheres increased in the microspheres that were initially in 2 mM CaCl 2 and decreased in those which were at 1 mM CaCl 2. (2) When these microspheres were injected subcutaneously in a rat, serum calcium rose from baseline of 10.6 to 12.2 mg/dl. Conclusions. Calmodulin can be used to develop calcium sensitive PLGA microspheres for the delivery of PTH. Bioactivity of PTH is preserved with coencapsulation of calmodulin.