SUN-276 Pembrolizumab-Induced Secondary Adrenal Insufficiency Presenting as Severe Hyponatremia in an 80-Year-Old Male

Background Pembrolizumab is an anti-programmed death 1 (PD-1) antibody designed to incite an immune response against malignant cells. By virtue of this mechanism of action, its use has given rise to immune-related adverse events including those affecting the endocrine system. Adrenal insufficiency can occur rarely with anti-PD-1 therapy, and symptoms are usually non-specific. Clinical Case An 80-year-old male, known case of non-small cell lung cancer stage IV presented with a 2-week history of progressive body weakness with anorexia, shortness of breath and low-grade fever. He had just received the 4th cycle of pembrolizumab prior to the onset of symptoms. Past medical history was significant for hypertension and type 2 diabetes mellitus which were both controlled, and pulmonary tuberculosis with completed treatment. On physical examination, he was drowsy but oriented. He was normotensive (110/70 mmHg) and tachypneic (28 cpm) with rales on both lung fields. Baseline capillary glucose level was 107 mg/dL. Chest radiograph showed hazy opacities in the right upper to middle region. Blood chemistry revealed severe hyponatremia (114 mmol/L, NV 135-145 mmol/L) and low serum osmolality (247 mOsm/kg, NV 280-300 mOsm/kg). Random (taken 1230H) ACTH and cortisol were <5.00 pg/mL (NV <46 pg/mL) and 2.00 μg/dL (NV 4.30-22.40 μg/dL), respectively. Such levels were judged to be low in the background of an acute illness. Thyroid function tests were normal – TSH 0.993 μIU/mL (NV 0.55-4.78 uIU/mL), FT3 3.890 pg/mL (2.30-4.20 pg/mL), FT4 1.450 ng/dL (0.89-1.76 ng/dL). Magnetic resonance imaging of the pituitary gland did not show abnormal parenchymal enhancement or enlargement. Pembrolizumab-induced secondary adrenal insufficiency was the most probable cause of the hyponatremia. He was started on IV hydrocortisone, as well as piperacillin-tazobactam for pneumonia. Oxygen support via nasal cannula was given. Feeding via nasogastric tube was decided to ensure nutrition and prevent aspiration. He was transferred to the intensive care unit for careful monitoring. Serum sodium level was corrected gradually, with marked clinical improvement thereafter. Within 48 hours, he was transferred to regular room and oral feeding commenced. Hydrocortisone was shifted to prednisone on discharge, with steroid tapering schedule and close follow-up with endocrinologist advised. Conclusion We presented a case of secondary adrenal insufficiency which likely resulted from hypophysitis induced by pembrolizumab. Hypophysitis following anti-PD1 treatment occurs in <1% of patients on immunotherapy. In such cases, ACTH deficiency is usually isolated and pituitary imaging is frequently normal. Since more patients are being placed on immune-checkpoint inhibition, clinicians should be vigilant for these adverse events, particularly the endocrinopathies which may have non-specific symptoms and may be irreversible.