Drug Interactions between octreotide and Vitamin D3

MONITOR: Due to their gastrointestinal pharmacologic effects, somatostatin analogs (e.g., octreotide, lanreotide) may variously affect the absorption of dietary nutrients and concomitantly administered oral medications. Somatostatin analogs have been shown to prolong gastrointestinal transit time and inhibit intestinal absorption of some nutrients such as fat. Clinical data are limited, however. In case reports, octreotide has been reported to reduce the relative bioavailability of cyclosporine. Transplant rejection and significant reductions in cyclosporine levels, sometimes to undetectable levels, have been reported in association with the interaction. Vitamin K absorption was not affected when concomitantly administered with lanreotide according to the manufacturer.

MANAGEMENT: Clinicians should be aware of the potential for altered absorption of concomitantly administered oral medications during treatment with somatostatin analogs. Blood levels and clinical response should be monitored, particularly for drugs that have a narrow therapeutic index, and the dosages adjusted as necessary.

MONITOR: Additive effects and possible toxicity (e.g., hypercalcemia, hypercalciuria, and/or hyperphosphatemia) may occur when patients using vitamin D and/or vitamin D analogs ingest a diet high in vitamin D, calcium, and/or phosphorus. The biologically active forms of vitamin D stimulate intestinal absorption of calcium and phosphorus. This may be helpful in patients with hypocalcemia and/or hypophosphatemia. However, sudden increases in calcium or phosphorus consumption due to dietary changes could precipitate hypercalcemia and/or hyperphosphatemia. Patients with certain disease states, such as impaired renal function, may be more susceptible to toxic side effects like ectopic calcification. On the other hand, if dietary calcium is inadequate for the body's needs, the active form of vitamin D will stimulate osteoclasts to pull calcium from the bones. This may be detrimental in a patient with reduced bone density.

MANAGEMENT: Given the narrow therapeutic index of vitamin D and vitamin D analogs, the amounts of calcium, phosphorus, and vitamin D present in the patient's diet may need to be taken into consideration. Specific dietary guidance should be discussed with the patient and regular lab work should be monitored as indicated. Calcium, phosphorus, and vitamin D levels should be kept within the desired ranges, which may differ depending on the patient's condition. Patients should also be counseled on the signs and symptoms of hypervitaminosis D, hypercalcemia, and/or hyperphosphatemia.