"Determining the Maximum Tolerable Dose and Pharmacokinetic Parameters of Intravenous PTHrP(1-36)"

Osteoporosis is a growing health problem. The most commonly used treatment options are anti-resorptive agents, which give a bone density increase of the lumbar spine in the 6-8% range over a 3-5 year period. A 100 % increase in bone mass would be needed to restore bone mass to peak, premenopausal levels. The ideal drug to treat osteoporosis would be a pure skeletal anabolic agent; however, the only currently approved anabolic agent, parathyroid hormone [PTH(1-34)] appears to only increase lumbar spine mass by 12-15% over a 2-3 year period when given alone.

Parathyroid hormone-related protein, [PTHrP(1-36)], is a peptide secreted by almost all normal tissues and cells that shares significant homology with Parathyroid Hormone (PTH); and the investigators believe that it shares anabolic properties with PTH. In a previous study, PTHrP(1-36) administered subcutaneously to postmenopausal women for two weeks increased markers of bone formation (osteocalcin) while decreasing markers of osteoclastic bone resorption. In a subsequent study, postmenopausal osteoporotic women on estrogen received 6.56 mcg/kg PTHrP(1-36) subcutaneously for 3 months daily. They experienced a 4.7% increase in bone mineral density (BMD) of the lumbar spine compared to those taking placebo. The also displayed an increase in serum osteocalcin, a marker of bone formation, with no change in several markers of bone resorption. As a result of both rat and human research studies, the investigators feel the rapid absorption and clearance of PTHrP(1-36) likely plays a central role in its anabolic effect in humans.

In order to further assess the absorption of PTHrP, the investigators are doing this pharmacokinetic study using bolus intravenous PTHrP. It combines both pharmacokinetic and dose escalation methods for studying intravenous PTHrP given via a one-time bolus injection. Subcutaneous PTHrP has been shown to increase bone mass in previous studies done and would be the preferred route for administration of PTHrP in the treatment of osteoporosis. Comparing intravenous with subcutaneous PTHrP will yield much information about the bioavailability of PTHrP in vivo.

This six hour study has been designed to achieve the goals of (1) determining what is the maximum safe dose of PTHrP(1-36) that can be given intravenously as a one time bolus dose; and (2) what is the pharmacokinetic profile of a one-time intravenous bolus dose of PTHrP. A placebo group of 5 subjects will initially serve as a control. The investigators feel it is important to begin with very small amounts of intravenous PTHrP and gradually increase the dose for safety reasons. The initial single bolus intravenous dose to be tested is intentionally low (4 micrograms). Subsequent doses of PTHrP will be gradually increased using a standard dose escalation scale in groups of three subjects until a maximal dose of 400 mcg is reached or a dose limiting toxicity occurs at a dose lower than 400 mcg. The maximum determined safely tolerated dose will then be administered to a total of 10 normal healthy volunteers.

In all subjects at different dosing groups, frequent blood sampling within the first 20 minutes after IV administration of PTHrP will yield pharmacokinetic data about peak PTHrP blood levels and time to peak level T(max). After research subjects receive an intravenous bolus of PTHrP; blood samples will be collected for pharmacokinetic analysis of PTHrP at intervals of 2 minutes for the first 20 minutes, than at 25, 30, 45 and 60 minutes, then hourly for the next five hours. Measurement of calcium, creatinine, phosphorus, PTH, 1,25 Vitamin D and markers of bone metabolism will be obtained at baseline, and at hours one and three. After the baseline urine sample is obtained, urine will be collected at two hour intervals for calcium, creatinine, phosphorus, and markers of bone metabolism.