Discussion

Mineralization of vasculature and various soft tissues (systemic mineralization) was observed in toxicology studies in rats in a time- and dose-dependent manner. This change was consistent with the presence of calcium-phosphorus deposition within the vascular wall and parenchyma of tissues such as the stomach, kidney, aorta, and heart. The stomach appeared to be the most sensitive tissue, since mineralization of gastric fundic mucosa occurred prior to the onset of mineralization in other tissues. Male rats were consistently more sensitive to this toxicity than were female rats. In the pivotal one-month toxic-ity study in rats, the no-effect level for systemic mineralization was 0.1 mg/kg (0.6mg/m2) in males and 0.3 mg/kg (1.8mg/m2) in females, which were associated with PD0325901 steady-state plasma AUC(0-24) values of 231 and 805ng-h/mL, respectively. Systemic mineralization was not observed in dogs or monkeys, despite pharmacologic inhibition of tissue pMAPK levels (>70%), administration of lethal doses, and exposures greater than 10-fold of those that induced mineralization in rats (10,600ng-h/mL in dogs and up to 15,000 ng • h/mL in monkeys). Systemic mineralization was not observed in mice despite administration of PD0325901 at doses up to 50 mg/kg (150mg/m2).

Systemic mineralization observed in rats following administration of PD0325901 is consistent with vitamin D toxicity due to dysregulation in serum calcium and phosphorus homeostasis (Grant et al., 1963 - Rosenblum et al., 1977- Kamio et al., 1979- Mortensen et al., 1996- and Morrow, 2001). A proposed hypothesis for the mechanism of this toxicity is depicted in Figure 3. Elevated serum phosphorus levels (hyperphosphatemia) and decreased serum albumin were observed consistently in rats administered PD0325901. Although serum albumin levels are decreased in rats treated with PD0325901, calcium values typically remain unchanged or slightly elevated in these animals, indicating that free, non-protein-bound calcium is increased (Rosol and Capen, 1997; Payne et al., 1979; Meuten et al., 1982). Decreased parathyroid hormone levels (PTH) were observed in the rat studies. PTH plays a central role in the hormonal control of serum calcium and phosphorus. PTH is produced by the parathyroid gland and induces conversion of 25-hydroxyvitamin D (which is produced in the liver) to 1,25-dihydroxyvitamin D (calcitriol) in the kidney. 1,25-Dihydroxyvitamin D elicits increased absorption of calcium from the gastrointestinal tract. In addition, PTH mobilizes calcium and phosphorus from bone by increasing bone resorption, increases renal absorption of calcium, and increases renal excretion of phosphorus (in order to regulate

GI Tract

| 1, 25-Dihydroxyvitamin D

25-Hydroxyvitamin D

Figure 3 Hypothesis for the mechanism for systemic mineralization in the rat following PD0325901 administration.

GI Tract

| 1, 25-Dihydroxyvitamin D

25-Hydroxyvitamin D

Figure 3 Hypothesis for the mechanism for systemic mineralization in the rat following PD0325901 administration.

serum phosphorus levels). Elevations in serum calcium typically elicit decreased PTH levels as a result of the normal control (negative feedback loop) of this endocrine system (Rosol and Capen, 1997). The decreases in PTH observed in the rats were believed to be due to the elevations in serum calcium (hypercalcemia). Hyperphosphatemia in the presence of normo- or hypercalcemia can result in an increased Ca x P product, which is associated with systemic mineralization (Block, 2000). Hyperphosphatemia was also observed in rats administered PD176067, which is a reversible and selective inhibitor of fibroblast growth factor receptor tyrosine kinase. In these animals, vascular and soft tissue mineralization also occurs (aorta and other arteries, gastric fundic mucosa, myocardium, renal tubules), probably due to increased Ca x P product (Brown et al., 2005b).

Administration of PD0325901 to rats resulted in significantly increased levels of plasma 1,25-dihydroxyvitamin D. The mechanism for this action is not known but is not believed to be due to a metabolite of PD0325901. This is the most potent form of vitamin D and the primary metabolite responsible for regulating serum calcium and phosphorus. Vitamin D is converted to 25-hydroxyvitamin D in the liver and then 1-hydroxylated to 1,25-dihydroxyvi-tamin D in renal tubules. 1,25-Dihydroxyvitamin D acts by increasing absorption of calcium and phosphorus from the gastrointestinal tract, and can increase calcium and phosphorus reabsorption by renal tubules. Hyperphosphatemia and increased plasma 1,25-dihydroxyvitamin D levels in rats occurred 1 to 2 days prior to the detection of tissue mineralization at doses <3 mg/kg (18mg/m2).

Administration of PD0325901 to rats resulted in bone lesions that included necrosis of the metaphysis and the ossifying zone of the physis, and thickening of the zone of hypertrophying cartilage of the physis. The expansion of chon-drocytes in the physis may be a response to the metaphyseal necrosis and loss of osteoprogenitor cells. These changes are characterized by localized injury to bone that appear to be due to local ischemia and/or necrosis. Skeletal vascular changes may be present in these animals, resulting in disruption of endochondral ossification. Skeletal lesions, including bone necrosis, can result from vitamin D intoxication (Haschek et al., 1978). The skeletal lesions observed in rats administered PD0325901 are similar to those reported with vitamin D toxicity, which provides additional evidence that toxicity occurred via induction of 1,25-dihydroxyvitamin D. Bone lesions similar to those observed in rats were not seen in dogs, monkeys, or mice administered PD0325901.

In summary, PD0325901-induced systemic mineralization in the rat results from a dysregulation in serum phosphorus and calcium homeostasis. This dysregulation appears to result from toxicologically significant elevations in plasma 1,25-dihydroxyvitamin D levels following drug administration. Based on the toxicology data, rats are uniquely sensitive to this toxicity. A summary of the primary target organ toxicities observed in the preclinical studies is presented in Table 5. Toxicity to the skin (epidermal lesions) and gastrointes-

TABLE 5 Primary Target Organ Toxicities Observed in Preclinical Studies

Organ System

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment