P3875 - A Case of Prolonged Generalized Weakness in a Patient With Metastatic Rectal Cancer 6 Weeks After Last Dose of Panitumumab

Barundeep Singh, MBBS, Vivek Hande, MBBS, MD, R Ananthakrishnan, MBBS, MD, DM, Amol N. Patel, MBBS, MD, DM, Kaushik MR, MBBS, MD, DM
INHS Asvini, Colaba, Mumbai, Maharashtra, India

Introduction: Metastatic colorectal cancer presents various clinical challenges. We report a case of prolonged generalized weakness without cachexia.

Case Description/Methods: A 64-year-old man with metastatic rectal cancer treated with FOLFOX (folinic acid, fluorouracil, oxaliplatin) and Panitumumab completed 10 chemotherapy cycles. He then had mild COVID-19 infection, followed by generalized muscular weakness, mainly in legs, difficulty walking, and muscle cramps. He also had chronic constipation and pain while defecating, taking oral laxatives.

On examination, HR: 108 bpm, dry tongue, proximal leg weakness with power score of 3/5 bilaterally, and anal fissures. No signs of cancer cachexia were present.

Investigations and Management:

RBS: 116 mg/dl. ECG: ST depression, T-wave inversion in inferior leads, prominent U waves, and prolonged QT interval. S. Potassium: 2.2 mEq/L, Serum albumin: 2.9 mg/dl. Serum anion gap, CKP, and other labs were normal.

Considering b/l leg weakness after recent Covid-19 infection, initial DDs included Guillain-Barré syndrome (ruled out by absence of ascending weakness and areflexia), myasthenia gravis (ruled out by lack of fatigable weakness), and spinal cord compression due to metastasis (ruled out by normal MRI spine).

Given the weakness, recent laxative use, ECG changes, and hypokalaemia, potassium deficit was suspected. Initial management involved oral and parenteral potassium replacement. Despite interventions, symptoms and low potassium persisted. His records showed no myopathy causing drugs or chronic alcohol abuse. His last Panitumumab dose was six weeks before weakness onset.

Further assessments revealed low serum magnesium and renal potassium wasting, with urine creatinine to potassium ratio: 130 mEq/gram. He received significant potassium (127.5 grams) and magnesium (71 grams) replacement.

Discussion: Serum potassium and magnesium levels stabilized over 36 days of hospitalization. Weakness dissipated, with normal neuro-muscular examination at discharge. Panitumumab (Vectibix), an IgG2k monoclonal antibody targeting EGFR, is FDA approved for metastatic colorectal cancer with disease progression. It can cause hypokalaemia, hypomagnesemia, hypocalcemia, and other electrolyte disturbances via renal toxicity and ion channel effects.

Conclusion:

This case highlights the need for vigilant monitoring and replacement of serum potassium and magnesium in metastatic colorectal cancer patients on Panitumumab, extending from initiation of agent to at least 8 weeks post-therapy.


Disclosures:


Barundeep Singh, MBBS, Vivek Hande, MBBS, MD, R Ananthakrishnan, MBBS, MD, DM, Amol N. Patel, MBBS, MD, DM, Kaushik MR, MBBS, MD, DM. P3875 - A Case of Prolonged Generalized Weakness in a Patient With Metastatic Rectal Cancer 6 Weeks After Last Dose of Panitumumab, ACG 2024 Annual Scientific Meeting Abstracts. Philadelphia, PA: American College of Gastroenterology.