Acute toxicological evaluation of bone-targeted parathyroid hormone-related peptide (PTHrP) antagonist to inhibit breast cancer metastases to bone.

The present study evaluates the acute toxicological profile and drug distribution of a novel bone-targeted parathyroid hormone-related peptide (PTHrP) antagonist for bone metastatic breast cancer. [W2]PTH(1-33)-CBD was created by fusing PTHrP antagonist peptide to the bacterial collagen-binding domain (CBD) of ColG collagenase from Clostridium histolyticum to target the drug to type 1 collagen in bone. Acting as an inverse agonist at the PTH/PTHrP receptor, [W2]PTH(1-33)-CBD induced apoptosis in breast cancer cells and reduced tumor burden in mouse tibia. Female C57BL/6 mice were injected with vehicle, 320 µg/kg, or 1000 µg/kg [W2]PTH(1-33)-CBD as a single injection. Animal behavior, body weight, mortality, and biochemical and organ toxicity assays showed no significant changes during 28-day study period. [W2]PTH(1-33)-CBD distribution assessed by immunohistochemistry confirmed localization to bone, skin, and kidney. No significant changes in liver enzymes or serum protein were observed. Treatment reduced serum calcium and increased creatinine, but kidney histology showed no toxicity. Histological analysis of the kidneys showed no alterations, indicating no toxicity. Histological analysis of spleen, lungs, skin, and bone showed no pathological changes. Pharmacokinetic analysis was done after a single injection of 1000 µg/kg dose (via subcutaneous and intravenous) to female Sprague Dawley rats to determine how the body metabolizes [W2]PTH(1-33)-CBD. Serum was collected at various time points, and drug analysis was performed. Exposure from both routes was similar, indicating complete absorption of [W2]PTH(1-33)-CBD following SC. No clinically significant biochemical or histopathological changes were observed at these doses, thus establishing a safety profile for treatment with novel [W2]PTH(1-33)-CBD.