Management of Rhabdomyolysis in a Patient Treated with Clozapine: A Case Report and Clinical Recommendations

Recurrent high creatine kinase levels under clozapine treatment - a case report assessing a suspected adverse drug reaction

Suspected adverse drug reactions (ADRs) during treatment with clozapine often prompt therapeutic drug monitoring (TDM) in clinical practice. Currently, there is no official recommendation for pharmacogenetic (PGx) testing in the context of clozapine therapy. In this case report, we demonstrate and discuss the challenges of interpreting PGx and TDM results highlighting the possibilities and limitations of both analytical methods. A 36-year-old male patient with catatonic schizophrenia was treated with clozapine. He experienced multiple hospitalizations due to elevated creatine kinase (CK) levels (up to 9000 U/L, reference range: 30-200 U/L). With no other medical explanation found, physicians suspected clozapine-induced ADRs. However, plasma levels of clozapine were consistently low or subtherapeutic upon admission, prompting us to conduct a PGx analysis and retrospectively review the patient's TDM data, progress notes, and discharge reports. We investigated two possible hypotheses to explain the symptoms despite low clozapine plasma levels: Hypothesis i. suggested the formation and accumulation of a reactive intermediate metabolite due to increased activity in cytochrome P450 3A5 and reduced activity in glutathione S-transferases 1, leading to myotoxicity. Hypothesis ii. proposed under-treatment with clozapine, resulting in ineffective clozapine levels, leading to a rebound effect with increased catatonic symptoms and CK levels. After considering both data sources (PGx and TDM), hypothesis ii. appeared more plausible. By comprehensively assessing all available TDM measurements and examining them in temporal correlation with the drug dose and clinical symptoms, we observed that CK levels normalized when clozapine plasma levels were raised to the therapeutic range. This was achieved through hospitalization and closely monitored clozapine intake. Therefore, we concluded that the symptoms were not an ADR due to altered clozapine metabolism but rather the result of under-treatment. Interpreting TDM and PGx results requires caution. Relying solely on isolated PGx or single TDM values can result in misinterpretation of drug reactions. We recommend considering the comprehensive patient history, including treatment, dosages, laboratory values, clinic visits, and medication adherence.

Toxicity and Adverse Effects in Clozapine-Related Presentations to a Medical Toxicology Service in Western Sydney

BACKGROUND

Clozapine is an anti-psychotic agent, reserved for treatment-resistant schizophrenia, with demonstrated efficacy in an otherwise therapeutically challenging patient population. We aimed to review the full spectrum casemix of clozapine presentations to our tertiary toxicology service.

METHODS

In this retrospective study, we reviewed consecutive clozapine related toxicity presentations to a tertiary medical toxicology inpatient and consultation service-including deliberate self-poisoning (DSP), adverse drug reaction (ADR), recreational use, and therapeutic misadventure over a 10-year period from 2011 to 2021. Data were extracted for demographics, ingested dose, exposure characteristics, and patient outcome.

RESULTS

We identified 83 patients with clozapine-related presentations over the 10-year period. Twenty-two patients were excluded. Of the remaining 61 patients, 28 patients presented with DSP, 20 patients with accidental overdose, and 13 patients with an ADR; no patients presented with recreational use. It was noted that ADRs were largely idiosyncratic reactions and not always related to dose adjustments. In the context of therapeutic misadventure and DSP, we noted that a lower mean dose achieved a higher poison severity score (PSS) in clozapine-naive patients when compared to those patients on regular clozapine.

CONCLUSIONS

The presentation of clozapine-related toxicity differs depending on the modality of ingestion, whether DSP, accidental, or as a result of ADR. Patients naive to clozapine therapy tend to experience higher PSS with lower doses ingested either in a deliberate self-poisoning or accidental ingestion context. This is likely due to tolerance to the sedative properties of clozapine. No patients manifested clinical toxicity greater than 8 hours after ingestion, with an observation period of 6 hours accurately identifying toxicity in most patients.