Clozapine and Respiratory Infection Risk: What to Know

Clozapine is considered the drug of choice for treatment-resistant schizophrenia in guidelines globally, but it remains significantly underutilized. This is largely due to its range of side effects, particularly its increased infection risk which prompted the US Food and Drug Administration (FDA) to mandate regular blood testing to monitor neutrophil counts.

The COVID-19 pandemic raised new concerns about the care of clozapine-treated patients, leading clinicians and patients to urge the FDA to relax prescription requirements for the drug under the Risk Evaluation and Mitigation Strategy (REMS) program.

As the FDA prepares for a public hearing in November on proposed adjustments to the drug’s REMS criteria, a growing body of research is challenging the previous understanding of clozapine and infection risk.

Clarifying the Risk

Research on the link between clozapine and respiratory infections has produced conflicting results. Some studies indicate little to no increased risk for mild COVID-19 and other respiratory illnesses, while others have shown a higher likelihood of severe infection.

A recent nationwide Danish registry study of respiratory infections in people with a schizophrenia spectrum disorder could bring some clarity, Maxime Taquet, MD, a clinical lecturer at the University of Oxford, Warneford Hospital, Oxford, England, told Medscape Medical News.

By tracking periods when patients were on and off clozapine and other antipsychotics, the study offers more precise risk estimates, distinguishing the risks associated with the antipsychotic from those related to underlying schizophrenia, said Taquet, who authored an accompanying editorial on the study.

“It’s very important to try to disentangle the effects of schizophrenia, its severity, from the medication,” Taquet said. “I think that the Danish study is the first to try and really do that with as much precision as possible.”

After adjusting for key confounders including economic status and COVID-19 vaccination status, the researchers found that individuals taking antipsychotics had lower odds of testing positive for SARS-CoV-2 and similar rates of filled anti-infective prescriptions as those not taking the drugs.

Although antipsychotic use was not linked to higher rates of mild infection, it was linked to an increased risk for COVID-19 hospitalization in individuals older than 70 years, as well as hospitalization and death from other respiratory infections, mainly pneumonia, in those older than 40 years.

Notably, there was no excess risk for any outcome with clozapine vs other antipsychotics.

Strong Link to Pneumonia Risk

Results from a longitudinal Finnish study, just published in The American Journal of Psychiatry, also show an increased risk for severe outcomes from ileus and pneumonia among more than 2600 patients with schizophrenia taking clozapine.

Twenty years after initiating clozapine, the cumulative incidence estimate for ileus was 5.3% — more than sixfold higher than previously reported. The incidence of pneumonia was also high, at 29.5%.

Both illnesses were significantly associated with mortality, with odds ratios of 4.5 and 2.8, respectively.

These findings align with previous pharmacovigilance studies, with reported mortality rates for gastrointestinal hypomotility and pneumonia that were 4-10 times higher than those for agranulocytosis, the researchers said.

The study “really adds to a growing body of research suggesting a connection between clozapine use and a higher risk of developing pneumonia,” Robert O. Cotes, MD, a professor of psychiatry and behavioral sciences at Emory University, Atlanta, who specializes in the use of clozapine, told Medscape Medical News.

“Additionally, when people on clozapine do contract pneumonia, there’s concern the condition may be more dangerous,” he added.

A Closer Look at Neutropenia Risk

Neutropenia receives the lion’s share of attention among clozapine’s potential side effects, but this focus may need to be re-evaluated, Cotes said.

He pointed out that recent data suggest the risk for severe neutropenia, 2-3 years after initiating clozapine, is comparable to that of other antipsychotics.

A 2023 study of 26,630 clozapine users in Australia and New Zealand showed that most cases of severe neutropenia leading to clozapine cessation peaked within 18 weeks and was negligible after 2 years. This suggests weekly hematologic monitoring could potentially be discontinued after the 2-year mark.

Another study reported earlier this year by Medscape Medical News showed a low risk for mild or moderate neutropenia and no severe cases in nearly 1000 people taking clozapine.

“I worry that we may be missing the forest for the trees by hyperfocusing on neutropenia and not considering clozapine’s other potential serious side effects like pneumonia, myocarditis, and gastrointestinal hypermotility,” Cotes said.

Importance of Vaccines

The findings of these studies highlight the importance of vaccines in this at-risk group, said Taquet, a point emphasized by investigators of the Danish study he reviewed.

“Inspired by the experience of COVID-19 vaccine prioritization in severe mental illness and based on our findings, there is momentum for preventive action,” the authors wrote. “Our findings do not suggest the avoidance of specific antipsychotics but rather a call for increased vigilance regarding this at-risk group.”

This includes recommending pneumococcal, influenza, COVID-19, and other anti-infective vaccines in those older than 40 years treated with, or due to start, an antipsychotic.

“It’s not mandatory, but we do recommend that patients on clozapine get the regular vaccines,” Taquet said.

Pointing to the recent study on pneumonia risk, Cotes said addressing underlying risk factors, such as smoking, obesity, and possibly sedation and excessive salivation caused by clozapine, is key.

“And to make sure that vaccinations are up to date, particularly heading into this fall,” he added.

Rethinking Clozapine REMS

One of the most challenging issues facing clinicians and researchers is how to help people understand the safety profile of clozapine and to use it with more confidence, Coates said.

“A lot of people hear about clozapine and they think about neutropenia, they think about side effects, the REMS system, and all of these factors really drive down clozapine utilization,” he said.

Treatment-resistant schizophrenia affects about a quarter of those with schizophrenia, yet only 4% of these patients receive clozapine in the United States, Coates said. That number may be even lower for its other indication of reducing suicidal behavior in patients with schizophrenia or schizoaffective disorder.

The clozapine REMS is viewed as a major barrier to utilization and requires certification of pharmacists and physicians and use of a central system to monitor absolute neutrophil counts for neutropenia in patients.

As previously reported by Medscape Medical News, in November 2021, the FDA opted to temporarily exercise enforcement discretion for certain aspects of the drug safety program to ensure continuity of care for patients after concerns were raised by the American Psychiatric Association (APA) along with other professional organizations.

Even with that temporary enforcement discretion, “reports have shown that over half of those prescribed clozapine have trouble accessing the medication because of the REMS program,” a spokesperson for the APA told Medscape Medical News.

“Not only are patients having trouble accessing the medication, many have trouble finding a prescriber in their geographic locations and others because of the monitoring requirements have their treatment discontinued leading to negative outcomes,” the spokesperson said.

The FDA is currently reviewing the clozapine REMS and is holding a joint advisory committee meeting on November 19 to discuss the review and “possible changes to minimize burden on patients, pharmacies, and prescribers while maintaining safe use of clozapine.”

The APA plans to submit written and oral comments to the advisory committees.

“We are hopeful that the re-evaluation meeting in November will remove barriers and increase access to clozapine, which is currently highly underutilized, especially in marginalized communities,” the spokesperson said.

Cotes reported serving as a speaker and consultant for Saladax Biomedical and as a consultant for Syneos Health. Taquet reported having no competing interests.