Effect of octreotide LAR dose and weight on octreotide blood levels in patients with neuroendocrine tumors

Objectives: Octreotide long acting repeatable (LAR) is widely used for the control of symptoms of functional neuroendocrine tumors. At doses of 30 mg/mo, up to 40% of patients require subcutaneous octreotide rescue'' and up to 40% of patients are given more than 30 mg of LAR/mo. Octreotide acetate binds to the sst2 receptor with an affinity (K-d) of approximately 1 x 10(-9) mol/L (congruent to 1000 pg/mL), but higher (congruent to 10,000 pg/mL) concentrations of octreotide are required to completely saturate this receptor. Octreotide blood level measurement may be useful to guide LAR therapy in symptomatic patients or in patients who have tumor growth on traditional LAR doses. We hypothesize that LAR doses of 60 mg/mo will produce blood levels of 10,000 pg/mL or greater. At identical monthly LAR doses, patients with higher weights will require more medication to achieve similar plasma octreotide levels than individuals with lower body weights. Methods: Trough plasma, serum, urine, and saliva octreotide levels were obtained from 52 patients with carcinoid syndrome receiving 20 (n = 8), 30 ( n = 19), or 60 mg LAR/mo ( n = 10). Octreotide levels were determined by radioimmunoassay. Results: The mean +/- SD plasma octreotide levels for patients receiving 20, 30, or 60 mg LAR/mo were 2518 +/- 1020, 5241 +/- 3004, and 10,925 +/- 5330 pg/mL, respectively. Patient weight ( kilograms) was inversely related to plasma octreotide levels. There was a significant correlation between plasma octreotide levels and octreotide levels measured in urine, saliva, and serum. Conclusions: Frequent measurement of octreotide levels may be useful to guide octreotide therapy in patients with poorly controlled symptoms or those patients experiencing tumor growth.