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Oral mucositis following drug toxicity and interaction: case report A 62-year-old woman developed oral mucositis secondary to everolimus toxicity following concomitant administration of everolimus and verapamil. The woman was diagnosed with a renal mass in 2007. At that time, she refused surgery. In 2017, her general condition deteriorated. Examination revealed a thrombosis of left renal vein. Based on findings she was diagnosed with renal cell carcinoma stage IV (T3cN1M1). Her medical history was significant for asthma, urticaria, Bouveret-Hoffmann syndrome (paroxysmal tachycardia), diabetes, and hypothyroidism. She also had moderate hepatic impairment and her condition was stratified into the moderate liver dysfunction group (group C). She was receiving verapamil 120mg twice a day [route not stated], metformin, levothyroxine-sodium [levothyroxine], telmisartan, desloratadine, tramadol, fluticasone-propionate [fluticasone], paracetamol, and tinzaparin-sodium [tinzaparin]. Based on her condition she started receiving oral everolimus at a reduced dosage of 7.5mg/day. Pharmaceutical consultation suspected interaction between the verapamil and everolimus. It was very difficult to replace verapamil due to her cardiac history. It was scheduled to assess her trough everolimus blood concentration at about 10 days after the started of treatment. She started everolimus on 28 September 2017 and quickly developed canker sores and lingual edema, which were aggravated progressively. On 6 October 2017 she consulted general practitioner. The woman started receiving sodium bicarbonate, nystatin, amphotericin B and lidocaine mouthwashes suspecting grade 3 oral mucositis. On 9 October 2017 the toxicity test was performed. Sampling was performed at 10:45am, the last intake of everolimus was on the previous day around noon. The electrochemiluminescence immunoassay (ECLIA) test revealed a residual everolimus plasma concentration at 52.4 ng/mL. Her everolimus levels were twice the toxic threshold level and 4.4 times the effective minimum threshold. Thus, oral mucositis due to everolimus toxicity attributed by the drug interaction of everolimus with verapamil was established. Thus, everolimus was stopped on 12 October 2017. On 16 October she still had grade 3 oral mucositis. However, her condition was significantly improved and inflammation and canker sores were decreased. She started receiving mouthwashes based on sodium bicarbonate, sucralfate, amitriptyline, lidocaine and prednisolone. Consequently she had significant improvement. On 23 October 2017 everolimus was re-initiated at a dose of 5mg. Consultation was scheduled after 2 days. Her tolerance to treatment was very good. During the first month of treatment, she experienced asthenia due to recent weight loss. The everolimus efficacy remained limited for more than one year, and she had a partial tumor response to the 5mg dose. Strobbe G, et al. Advantages of everolimus therapeutic drug monitoring in oncology when drug-drug interaction is suspected: A case report. Journal of O