SAT220 Rare Case Of Pembrolizumab Induced Primary Hypoparathyroidism

Disclosure: S.G. Goud: None. A. Siddiqui: None. M. Alkhathlan: None. C.Y. Fan: None. Introduction: Immune checkpoint inhibitors such as pembrolizumab (Keytruda) used in the treatment of various cancers are associated with immune mediated endocrinopathies, among which thyroid dysfunction and pituitary hypophysitis are common, whereas hypoparathyroidism is rare. We report a case of primary hypoparathyroidism and hypocalcemia with pembrolizumab. Clinical Case: 58-year-old male with metastatic urothelial cell carcinoma, status post neoadjuvant chemotherapy with gemcitabine and cisplatin in 2017, followed by radical cystectomy with recurrence of metastasis in 2018, who was started on Keytruda every three weeks. After 10 months, he developed immune mediated hypothyroidism treated with levothyroxine. After 33 months on Keytruda, he developed hematochezia with constipation and intermittent abdominal pain. Colonoscopy showed findings consistent with autoimmune colitis; Keytruda was held and colitis was treated with tapering steroids. Surveillance imaging showed stable disease, so Keytruda continued to be held. After 7 months of holding Keytruda, on routine blood work he had a critically low calcium level of 5.5 mg/dl (8.4-10.2 mg/dl) with tingling and numbness in his hands, muscle spasms, generalized weakness, and fatigue for the past few weeks. On exam there was no Trousseau’s sign, Chvostek's sign, or gait ataxia. Hemodynamically stable. Labs showed lowest corrected calcium of 5.5, normal renal function, low vitamin D of 8 ng/ml (30-100 ng/ml), elevated phosphorus of 6.3 mg/dl (2.5-4.5 mg/dl), and normal magnesium. PTH was frankly low at 12.7 pg/ml (15-65 pg/ml), indicating primary hypoparathyroidism. ACTH stimulation test was normal. Retrospective review of the labs from oncology office records showed gradual, progressive hypocalcemia for a few months prior to presentation, with calcium levels ranging between 6.7 to 7.7 mg/dl. Of note, calcium prior to pembrolizumab was normal at 9.4 mg/dl. He was treated with IV calcium gluconate and then transitioned to calcium carbonate (CaCO3) 500 mg TID, calcitriol 0.5 mg BID, and Vit D 50,000 IU twice weekly. Hydrochlorothiazide (HCTZ) added as calcium remained low. After three days calcium improved to 7 mg/dl range and he was discharged. Calcium levels checked periodically after discharge ranged from 8 to 9 mg/dl (8.4-10.2 mg/dl), and phosphorus levels normalized. CaCO3 dose was adjusted accordingly, Vit D was changed to 50,000 IU weekly based on a level of 71 ng/dl after 3 months, and HCTZ was discontinued after 6 months. PTH was still low at 9.7 pg/ml (15-65 pg/ml) 6 months after discharge, off pembrolizumab for over a year. Calcium sensing receptor (CaSR) activating antibody was not checked in our case. Conclusion: It is important clinicians be aware of the risk for hypocalcemia and its unpredictable presentation to help with timely recognition and management. Presentation: Saturday, June 17, 2023