Introduction
Over the past few months, new drugs have been approved for the treatment
of COVID-19 infection. Nirmatrelvir/ritonavir, approved in Spain in
March 2022 for the treatment of mild COVID-19 infections in high-risk
patients, has demonstrated an 89% reduction in the risk of progression
to hospitalization and death when treatment is initiated within the
first few days of symptom onset (1). The presence of ritonavir in this
drug causes a rapid and strong inhibition of cytochrome P450 (CYP3A4),
which represents the main pathway for the metabolism of a large number
of drugs. This interaction can become especially relevant in transplant
patients treated with tacrolimus, the main drug used in post-transplant
immunosuppressive therapy, since it is metabolized in the liver and
intestine by this enzyme (2). The interaction can cause elevations in
tacrolimus blood levels leading to adverse effects such as
nephrotoxicity or neurotoxicity.
In this study, we present the case of a kidney transplant patient who
suffered a large elevation of tacrolimus blood levels due to interaction
with nirmatrelvir/ritonavir.