Schmitt, Cassandra J

Abstract Purpose Clozapine is primarily metabolized by cytochrome P450 (CYP) 1A2 and to a lesser extent by CYP2C19, CYP2D6, and CYP3A4. The global pandemic of coronavirus disease 2019 (COVID-19) and use of nirmatrelvir/ritonavir brought forth new challenges for those caring for patients prescribed clozapine. Prescribing information describes a consideration for a clozapine dose reduction while monitoring for adverse reactions when clozapine is used with nirmatrelvir/ritonavir. Other drug information resources consider the combination a contraindication. A case is reported in which therapeutic drug monitoring demonstrated a lack of interaction between clozapine and nirmatrelvir/ritonavir. The complexities of infection-associated clozapine toxicity are discussed and a review is provided of CYP3A4 drug and gene interactions with clozapine. Summary A 75-year-old woman with schizophrenia, prescribed clozapine 200 mg by mouth at bedtime, was initiated on nirmatrelvir/ritonavir after developing a mild COVID-19 infection. Two days later, the patient was brought to the emergency department after an unwitnessed fall. Clozapine toxicity was included in the differential diagnosis, although no other signs or symptoms of elevated clozapine levels were present. Nirmatrelvir/ritonavir was discontinued. Clozapine was withheld for 1 day while a clozapine level was pending, but this measurement revealed no interaction when compared to a baseline level of clozapine. Conclusion Although there are hypothetical concerns when initiating a CYP3A4 inhibitor with concomitant clozapine, the information derived from this case does not clearly support an interaction with the addition of nirmatrelvir/ritonavir during mild COVID-19 infection. It is important to understand the risks of withholding clozapine, although fortunately no psychiatric concerns developed. In reviewing the literature, CYP3A4 inhibitors or genetic polymorphisms seem to have a minimal impact on clozapine levels, although clinical monitoring is warranted.