From Genotype to Guidelines: Rethinking Neutropenia Risk in Clozapine Use

Abstract

Clozapine, a second-generation antipsychotic known for its effectiveness in treating resistant schizophrenia, is often linked with serious hematological side effects, particularly neutropenia and agranulocytosis. This review investigates the underlying pathophysiological mechanisms of clozapine-induced neutropenia (CIN) and agranulocytosis (CIA), outlines associated risk factors, and evaluates current clinical management strategies. Clozapine’s pharmacological profile, marked by its antagonism of dopamine D4 and serotonin receptors, contributes to both its therapeutic advantages and hematological toxicity. Epidemiological data show a prevalence of CIN and CIA at approximately 3.8% and 0.9%, respectively, with onset typically occurring within the first six months of treatment. Key risk factors include older age, Asian and African American ethnicity, female sex, and certain genetic predispositions. The development of CIN and CIA may involve bone marrow suppression and autoimmune mechanisms, although the exact processes remain partially understood. Clinical presentation often includes nonspecific symptoms such as fever and signs of infection, necessitating regular hematological monitoring in accordance with established guidelines. Management strategies include dosage adjustments, cessation of clozapine, and the administration of granulocyte colony-stimulating factors (G-CSF). Advances in pharmacogenomics show promise for predicting susceptibility to CIN and CIA, potentially improving patient safety. This review emphasizes the importance of vigilant monitoring and personalized treatment approaches to reduce the risks associated with clozapine therapy.

1. Introduction

Schizophrenia is a debilitating psychiatric disorder with an estimated global prevalence of 1.4 to 4.6 cases per 1000 individuals and an incidence rate ranging from 0.16 to 0.42 per 1000 population [1]. These figures can vary depending on geographic region, population demographics, and differences in study design or diagnostic criteria. While antipsychotics remain the first-line treatment for schizophrenia, a subset of patients, classified as having treatment-resistant schizophrenia (TRS), fail to respond adequately. TRS is estimated to occur in approximately 22.8% of cases [2]. Clozapine remains the gold standard for the management of TRS due to its superior efficacy in reducing psychotic symptoms and preventing suicide in high-risk populations. Its broad receptor antagonism, particularly dopamine D4, serotonin 5-HT2A, muscarinic, histaminergic, and adrenergic receptors, contributes to both its clinical effectiveness and distinctive side effect profile. This receptor activity also explains its lower risk of extrapyramidal symptoms compared to first-generation antipsychotics.

Despite its clinical benefits, the use of clozapine is often limited by severe adverse effects, most notably hematological complications such as neutropenia (absolute neutrophil count [ANC < 1500 cells/mm³]) and agranulocytosis (ANC < 500 cells/mm³), which can be life-threatening. Clozapine was once temporarily withdrawn from the market in the 1970s due to reports of fatal agranulocytosis, but was reintroduced under strict monitoring protocols following evidence of its unmatched efficacy in TRS [3,4].

Although the exact mechanisms underlying CIN remain unclear, recent research has implicated genetic predispositions involving HLA-DQB1, HLA-B, and SLCO1B3/SLCO1B7 gene variants [4]. These findings are promising but remain in early stages of clinical validation and are not yet widely implemented in routine care. Additional risk factors include younger age, African American or Asian ancestry, and male sex. Although Individuals of African American ancestry have a higher risk and prevalence of neutropenia, it does not necessarily correlate with increased risk of CIN or CIA. Benign ethnic neutropenia (BEN) has been well documented for causing neutropenia in individuals of African descent. BEN is explored more in Section 4.

To mitigate these risks, clinical guidelines recommend routine monitoring of absolute neutrophil counts (ANC) throughout clozapine therapy. Regulatory frameworks, such as the now-retired Clozapine Risk Evaluation and Mitigation Strategy (REMS) program in the United States, have historically played a role in ensuring patient safety.

In this review, we examine current epidemiological data, clinical risk factors, and emerging genetic insights related to CIN. Particular attention is given to mechanisms involving immune-mediated pathways and gene variants that may inform future risk stratification. Understanding the biological basis of CIN not only enhances patient safety but may also allow for individualized treatment strategies, expanding safe access to this life-saving medication.

2. Pharmacology: Mechanism of Action

Clozapine is an atypical antipsychotic primarily used in cases of treatment-resistant schizophrenia. Its therapeutic effects are largely mediated through antagonism of dopamine and serotonin receptors. Clozapine antagonizes dopamine receptors D1–D5, although it exhibits a particularly high affinity for the D4 receptor [1,5,6]. In terms of serotonergic activity, clozapine interacts with the 5-HT1A and 5-HT2A receptors. At the 5-HT1A receptor, it acts as a partial agonist, a mechanism thought to contribute to improvements in negative and cognitive symptoms of schizophrenia [1,7,8]. Interestingly, clozapine’s effect on the 5-HT2A receptor diverges from that of classic GPCR antagonists; it induces receptor internalization, thereby functioning as a 5-HT2A functionally selective agonist. Its combined interactions with dopamine and serotonin receptors are believed to underlie its ability to reduce negative symptoms and minimize the occurrence of extrapyramidal symptoms (EPS) [1,2,9].

Beyond its dopaminergic and serotonergic actions, clozapine also exhibits significant activity at other receptor systems. It antagonizes muscarinic receptors M1, M2, M3, and M5; however, functional studies suggest that its action at M5 receptors may exhibit partial agonist or modulatory properties under specific conditions, reflecting a complex receptor interaction profile [1,2,10]. Notably, its interaction with the cholinergic receptors is nuanced. Clozapine demonstrates a complex pharmacodynamic profile on muscarinic receptors, primarily exhibiting antagonism at M1 and M3 but acting as a partial agonist at M4 receptors. According to studies by Michal et al. and Zorn et al., clozapine can act as a selective agonist at M4 receptors, which may contribute to both therapeutic efficacy and side effects such as cognitive changes and salivation [11,12,13]. This ambiguous binding behavior suggests that cholinergic modulation may underlie not only clinical benefits but also autonomic side effects such as tachycardia or constipation [14].

Additionally, clozapine blocks histamine H1 receptors and exhibits α1-adrenergic receptor antagonism. These interactions are associated with adverse effects, including hypotension, sedation, and reflex tachycardia [1,2].

Clozapine’s most deadly adverse effect, neutropenia/agranulocytosis, is thought to be caused by a multifactorial mechanism, some of which are postulated to involve genetic predisposition. The mechanisms behind neutropenia/agranulocytosis will be explored in depth in this paper’s Section 4. It remains unclear whether receptor-level effects modulate immune responses or hematopoiesis directly; however, the interplay between receptor binding and metabolic pathways may influence neutropenia risk and warrants further study.

3. Epidemiology and Risk Factors

CIN is defined as an absolute neutrophil count (ANC) below 1500 cells/mm³, while CIA refers to a more severe decline in neutrophil levels, with an ANC below 500 cells/mm³. Both conditions pose a substantial risk of infection and require prompt clinical intervention.

The prevalence of CIN is estimated at approximately 3.8%, with CIA reported in about 0.9% of clozapine-treated patients [15,16]. The highest risk period occurs within the first six months of therapy, with 84% of CIA cases developing within the first three months of treatment [15]. The risk of agranulocytosis ranges between 0.8% and 1.5% within the first year, with only a slight increase in cumulative risk over time [15,16]. While mortality from CIN is rare, with an incidence of 0.013%, fatal outcomes are commonly linked to CIA, with a reported case fatality rate of 2.1% [17].

A large-scale study conducted in the United Kingdom and Ireland reported CIN in 2.9% and CIA in 0.8% of patients, with incidence peaking between weeks 6 and 18 of treatment. Fatal agranulocytosis occurred in 0.03% of cases [18].

It is important to distinguish CIN from CIA, as these represent biologically and clinically distinct phenomena. CIN encompasses a spectrum of neutrophil reductions, most of which are transient and reversible, and many of which do not progress to clinically significant immunosuppression. CIA, by contrast, is characterized by profound and sustained neutropenia, carrying a substantially higher risk of serious infection and mortality. As highlighted by Silva et al., the majority of CIN cases do not progress to CIA, and conflating these categories may overestimate the perceived danger of clozapine treatment [19]. This distinction is supported by an Icelandic observational study, which found that only 2.9% (1 out of 34) patients who developed neutropenia subsequently progressed to agranulocytosis [20]. Understanding this distinction is critical for informed risk communication and for guiding appropriate monitoring and management decisions.

Identified risk factors for the development of CIN and CIA are somewhat variable across studies. Several reports, including Alvir et al., indicate that older age and female sex are associated with an increased risk of agranulocytosis [15]. However, other studies have identified younger age and male sex as potential risk factors, particularly for milder neutropenia or early-onset presentations [18,21]. This variability may reflect differences in study populations and genetic background. Ethnicity is a well-established factor influencing risk. African American ancestry is linked to both BEN and increased susceptibility to clozapine-induced neutropenia, particularly in individuals carrying the duffy-null allele [22]. Asian populations also exhibit elevated risk. A large retrospective cohort in China reported leukopenia in 3.9% and agranulocytosis in 0.21% of patients [23]. At the same time, a pharmacogenomic study in Japanese subjects identified similar genetic associations to those seen in Caucasian and African Ancestry groups [18,24]. These findings underscore the importance of considering ethnic background when assessing patient risk. However, current pharmacogenomic data remain heavily weighted toward European, African, and East Asian populations. Other ethnic groups, including Latin American, Middle Eastern, and Indigenous populations, are markedly underrepresented in large Genome Wide Association Studies (GWAS) [25]. Expanding genetic research to encompass these cohorts is critical to developing equitable, globally applicable clinical guidelines for clozapine use.

Moreover, studies have identified correlations between variants in HLA-DQB1 and HLA-B alleles [26,27], as well as polymorphisms in SLCO1B3/SLCO1B7, CYP1A2, FMO3, and cytokine genes involving TNF-α and IL-10 [28,29,30], suggesting a potential genetic predisposition to CIN. However, the clinical application of pharmacogenomic testing remains limited due to variability in findings, lack of standardized testing protocols, and the need for further validation in diverse populations.

4. Proposed Mechanisms of Action

The exact mechanism behind Clozapine causing neutropenia is multifactorial and not yet fully understood. Proposed pathways include both direct toxicity to myeloid precursors and immune-mediated reactions.

Clozapine is metabolized in the liver by cytochrome P450 enzymes to dimethyl-clozapine and clozapine N-oxide. These metabolites are relatively stable; however, clozapine metabolism via the flavin-containing monooxygenase-3 (FMO3) has been shown to produce the highly reactive nitrenium ion [31,32,33,34]. The Nitrenium ion is a highly electrophilic reactive intermediate capable of forming covalent adducts with cellular proteins. This interaction may disrupt neutrophil precursor function and lead to cellular apoptosis, contributing to hematologic toxicity [35,36,37]. Additionally, research has shown that the clozapine metabolite N-desmethylclozapine is more toxic than clozapine itself to myeloid precursor cells; however, the levels of N-desmethylclozapine required to induce toxicity in hematopoietic progenitor cells exceed typical serum concentrations found in clozapine-treated patients, making it a less likely primary mediator of CIN [38].

Neutrophils can utilize the NADPH/myeloperoxidase pathway and generate hypochlorous acid, which can oxidize clozapine to reactive metabolites that may covalently bind to neutrophils [39,40,41]. These interactions between metabolites and neutrophils may cause neutropenia or agranulocytosis via direct toxicity, initiating an immune response, or a combination of both. The rapid relapse of CIN and CIA upon rechallenge strongly supports an immunologic component, likely involving memory T-cell activation [32]. Conversely, nitrenium ion-mediated apoptosis of bone marrow precursors provides compelling evidence for a direct toxic pathway [38]. These mechanisms are not mutually exclusive and may act synergistically. Studies suggest that CIN may involve type II or type IV hypersensitivity reactions, although definitive characterization of the immune response remains incomplete. The presence of clozapine-modified self-antigens may promote autoantibody production or cytotoxic T-cell-mediated destruction of neutrophils and myeloid precursors [34,37,42].

Although current studies have not shown reliable and consistent findings, it is hypothesized that the production of the nitrenium ion plays a significant role in inducing neutropenia. Polymorphisms in NQO2, NAD(P)H quinone oxidoreductase 2, have been found to be associated with CIN [43]. NQO2 154AA has the strongest association of the studies’ polymorphisms with CIN [44].

Genome-wide association studies (GWAS), exome sequencing, and copy number variation (CNV) analyses have concluded in an association between HLA-DQB1, HLA-B (158T and 59:01), SLCO1B3, SLCO1B7, and ACKR1 with CIN/CIA [22,24,26,32,45,46,47].

Effect sizes associated with these genetic variants highlight their clinical significance. Athanasiou et al. reported an odds ratio (OR) of approximately 16.9 for the HLA-DQB1 6672G>C variant in association with CIA [47]. The CIAC GWAS identified HLA-DQB1 126Q as a risk variant with an effect size of 5.2–16.9 across different cohorts [32]. HLA-B variants 158T and 59:01 demonstrated respective effect sizes of 3.3 and 6.3–15.8 [32]. The SLCO1B3/SLCO1B7 variant rs149104283 was associated with an effect size of 4.32 [32]. ACKR1 rs2814778 was linked to a markedly increased risk of CIN, with an effect size of 20.36 [32]. These findings underscore the potential utility of pharmacogenomic risk stratification, though validation across diverse populations remains necessary.

HLA-DQB1 is the most well-known gene associated with CIN/CIA. The Clozapine-Induced Agranulocytosis Consortium (CIAC) performed a GWAS study that concluded that HLA-DQB1 with the amino acid polymorphism 126Q was most strongly associated with CIN in European populations [26,32]. Additionally, in the CLOZUK study, variant HLA-DQB1 6672G>C was also demonstrated to be associated with CIN/CIA. Although these studies highlight a correlation between these genetic variants and European populations, the exact mechanism behind HLA-DQB1 causing CIN remains unclear [32,45]. Furthermore, it is not clear if HLA-DQB1 also correlates with CIN/CIA in non-European populations [32,45].

Studies have shown an association between HLA-B and CIN/CIA, particularly with the amino acid polymorphism 158T [32]. HLA-B is an important component of the MHC class 1 antigen-presentation pathway, mediating the presentation of endogenous peptides to cytotoxic T cells [26]. HLA-B 158T has been previously associated with adverse drug reactions like Carbamazepine-induced Stevens-Johnson syndrome [48], and abacavir hypersensitivity syndrome [49]. Goldstein et al. studied the 158T variant of HLA-B and found that the threonine mutation suggested a much higher affinity for clozapine in comparison to other HLA-B variants [26]. Although the precise mechanism remains unclear, these findings suggest that specific HLA-B polymorphisms may predispose individuals to aberrant immune responses, contributing to the pathogenesis of CIN and CIA. These findings underscore the role of immune-related genetic factors in CIN/CIA, alongside emerging evidence for non-immune pathways such as solute carrier transporters.

SLCO1B3 and SLCO1B7 are part of the organic anion transporter family, which aids in transporting substances across cell membranes, most notably in the liver. SLCO1B3 and SLCO1B7 are crucial for the uptake of bile acids, bilirubin, and various drugs for metabolism and degradation. Legge et al. performed a meta-analysis and identified an intronic SNP, rs149104283, present in 7.37% of CIN cases in the CLOZUK sample [32,45]. This study may indicate that SLCO1B3 and SLCO1B7 may play a role in the hepatic clearance of clozapine; however, more studies are necessary to conclude. Notably, SLCO1B3 and SLCO1B7 have been linked to drug-induced adverse effects, including docetaxel-induced neutropenia and simvastatin-induced myopathy [32,50,51,52,53,54].

ACKR1 is the gene responsible for encoding the protein “Duffy antigen receptor for Chemokines” (DARC). Variants of ACKR1 have been associated with Plasmodium vivax resistance and benign neutropenia [55]. The variant of ACKR1, rs2814778, commonly known as the Duffy-null genotype, is common among individuals of African or Middle Eastern ancestry [56,57,58]. This variant is considered to be the cause of BEN [55]. Interestingly, a study by Legge et al. revealed that individuals with variant rs2814778 and taking clozapine were more likely to develop neutropenia (OR: 20.36) [22,32]. More research is necessary to identify the degree of clozapine’s impact in causing further neutropenia in individuals with BEN.

Beyond hematological toxicity, clozapine’s complex receptor binding profile may underlie several of its other serious side effects. For instance, its interaction with muscarinic receptors, particularly the M4 and M5 subtypes, has been linked to autonomic dysfunctions including tachycardia, hypersalivation, and constipation [11,12]. The paradoxical effects on cholinergic transmission may also contribute to cognitive impairment in some patients [1]. Meanwhile, its potent antagonism at histaminergic and adrenergic receptors may mediate sedation and orthostatic hypotension [2,41]. Though speculative, these pharmacodynamic properties may interplay with immune-mediated responses that potentially exacerbate or precipitate neutropenia, especially in genetically predisposed individuals [26]. Further studies are needed to clarify whether these receptor interactions contribute to hematological dysregulation either directly or via modulation of immune homeostasis [26].

5. Monitoring Guidelines, Risk Mitigation, and Treatment

Before initiating clozapine therapy, patients must undergo an absolute neutrophil count (ANC) assessment. Treatment is typically approved if the ANC exceeds 1500 cells/mm³ [59].

Monitoring protocols for CIN vary by country. The European Clozapine Task Force recommends weekly ANC monitoring during the first 18 weeks of therapy, followed by monthly monitoring for the subsequent 34 weeks. If no episodes of leukopenia or neutropenia occur within the first year, monitoring may be reduced to every 12 weeks during the second year and annually thereafter [59].

As of February 2025, the U.S. Food and Drug Administration (FDA) no longer requires participation in the Clozapine Risk Evaluation and Mitigation Strategy (REMS) program, which previously served as a coordinated monitoring system involving prescribers, pharmacies, and patients [59,60,61]. The program was discontinued following evidence that routine ANC monitor could be effectively maintained without the administrative infrastructure of REMS, and in response to concerns over its complexity and barriers to clozapine access [61,62].

Despite the dissolution of the REMS framework, routine ANC surveillance remains standard practice in the United States. Current U.S. recommendations include:

• Weekly ANC monitoring during the first 6 months of treatment [59].
• Biweekly monitoring during the second 6 months [59].
• Monthly monitoring after one year, for the duration of therapy [59].

Management strategies for CIN depend on the severity of neutropenia:

• Mild neutropenia (ANC: 1000–1499 cells/mm³): Clozapine may be continued, but ANC monitoring should be increased to three times per week [59].
• Moderate neutropenia (ANC: 500–999 cells/mm³): Clozapine should be temporarily discontinued. ANC should be monitored frequently until levels exceed 1000 cells/mm³, at which point treatment may be reinitiated [59].
• Severe neutropenia/agranulocytosis (ANC: <500 cells/mm³): Clozapine must be discontinued immediately. Reinitiation should be considered only on a case-by-case basis following a comprehensive hematologic evaluation [59].

These monitoring guidelines apply to patients whose baseline ANC exceeds 1500 cells/mm³. However, certain individuals exhibit BEN—most commonly associated with the Duffy-null Fy(a-b-) phenotype—which results in baseline ANC values below this threshold. This phenotype is particularly prevalent among individuals of African descent and Sephardic Jewish populations [63]. In these cases, clozapine initiation may be considered if the ANC exceeds 1000 cells/mm³ and the benefits outweigh the risks. While no standardized monitoring protocols exist for this population, initiating Granulocyte Colony Stimulating Factor (G-CSF) agents like filgrastim may be considered [64,65]. However, collaboration with hematology specialists is strongly encouraged to mitigate the risk of CIN or CIA [60,66].

In contrast to neutropenia, clozapine may also induce transient leukocytosis, particularly during the early stages of treatment. This hematologic effect is generally benign and may reflect increased granulopoiesis or a mild inflammatory response rather than infection [67]. While less clinically significant than neutropenia, clozapine-induced leukocytosis can complicate clinical interpretation. Recognizing this phenomenon may help prevent unnecessary diagnostic evaluations or premature discontinuation of therapy.

Additionally, concomitant medications such as valproate, carbamazepine, and lithium can influence neutrophil dynamics. Lithium has shown potential to elevate ANC in clozapine-treated patients, whereas carbamazepine may increase the risk of cytopenias and should be avoided [68,69,70].

From a clinical perspective, clozapine is most appropriate for individuals diagnosed with TRS who have failed to respond to at least two other antipsychotics. It may also be considered in patients with recurrent suicidal behavior or those experiencing severe aggression unresponsive to other agents [64]. However, its use should be carefully avoided or delayed in individuals with pre-existing hematologic abnormalities, significant cardiac disease, or a history of CIN or CIA.

In select cases, clozapine rechallenge after an episode of neutropenia may be considered, particularly when no alternative treatments are effective. Case reports, such as Shankar et al., have described successful rechallenges supported by adjunctive filgrastim to maintain ANC levels during treatment continuation. While G-CSF agents offer a valuable adjunct to clozapine rechallenge protocols, their prophylactic use remains an area of clinical debate. Risks associated with G-CSF administration include splenic rupture, bone pain, and rebound neutropenia [64,65,71].

In addition to monitoring ANC, clinicians should remain vigilant for other adverse effects, including myocarditis, seizures, constipation, metabolic syndrome, and orthostatic hypotension. Regular assessments of weight, lipid profiles, fasting glucose, CRP, and electrocardiogram are advised during the initial weeks of treatment. Distinguishing between clozapine-induced tachycardia and anxiety or psychotic agitation is also crucial, as highlighted by Pardossi et al. [72].

6. Future Scope and Limitations

This review highlights promising avenues for further research into the pathogenesis of CIN, particularly regarding genetic predispositions. Emerging evidence implicates genetic markers such as HLA-DQB1, HLA-B, SLCO1B3, SLCO1B7, and ACKR1 in increasing susceptibility to CIN. However, the clinical relevance of these associations may vary by population. For example, HLA-DQB1 variants have shown strong associations with CIN in individuals of European ancestry, but data supporting their impact in other ethnic groups remain limited. Future studies should aim to elucidate the molecular mechanisms by which these variants contribute to neutropenia and explore their distribution across diverse populations. Such efforts may help tailor antipsychotic regimens for individuals with treatment-resistant schizophrenia, enhancing both efficacy and safety.

In addition, expanding research on pharmacogenomics may facilitate the development of individualized monitoring protocols. Stratifying patients based on genetic risk could improve clinical outcomes while potentially reducing the burden of routine hematologic surveillance for low-risk individuals. Nonetheless, the implementation of pharmacogenomic testing in clinical practice faces challenges, including high costs, limited accessibility in resource-poor settings, concerns about genetic privacy, and potential patient resistance to testing. Additionally, despite robust genetic associations, pharmacogenomic testing has not been clinically utilized due to limited sensitivity, lack of validation, and uncertainty around its ability to significantly alter clinical outcomes. Current tests cannot yet reliably identify all at-risk individuals, making them insufficient as standalone tools for guiding treatment decisions. In addition to single-variant testing approaches, polygenic risk scores (PRS) are emerging as a promising tool to enhance predictive accuracy for CIN. PRS integrates the cumulative impact of multiple small-effect genetic variants, potentially offering a more refined individual risk estimate [73]. However, PRS development for CIN is still in its early stages and requires validation across diverse populations before it can be applied clinically.

While negative genetic test results may suggest a lower risk of CIN, they should not be used in isolation to exclude patients from hematologic monitoring. The sensitivity of current tests remains too low to ensure patient safety without concurrent ANC surveillance. Premature reliance on these tools could inadvertently increase the risk of undetected hematologic toxicity.

Reported testing sensitivity ranges from 11% to 36%, specificity from 89% to 98%, with a positive predictive value (PPV) of 5.1% to 25.9% and a negative predictive value (NPV) of 96.9% to 99.7% [32]. Notably, the highest sensitivity was observed in testing for HLA-DQB1 and HLA-B variants [26]. Despite its limitations, Girardin et al. proposed a cost-dependent clinical strategy: discontinuing intensive monitoring in patients lacking both HLA-DQB1 (126Q) and HLA-B (158T) alleles, thereby reducing clozapine-associated risk to levels comparable with other antipsychotics [74]. However, the feasibility of this approach hinges on the cost-effectiveness of genetic testing, which must fall below $700 per patient to be viable in the U.S. context [74,75]. It is also unclear whether this cost is applicable across different healthcare systems, particularly in low- and middle-income countries. Furthermore, while retrospective analyses support the concept, prospective studies are still needed to validate the safety and efficacy of pharmacogenomic-guided monitoring protocols in real-world clinical settings.

7. Materials and Methods

Search Strategy: To explore the genetic influence of clozapine to cause neutropenia, we conducted a comprehensive literature review using PubMed and Scopus. Although other databases may offer additional studies, our search was limited due to access constraints. Our search covered publications from 1990 to 2025 that included the phrases “clozapine induced neutropenia”, “clozapine induced agranulocytosis”, “treatment resistant schizophrenia”, “clozapine induced neutropenia monitoring guidelines”, and “clozapine induced neutropenia genes.” Clozapine was reintroduced to the market in the late 1980s, and its use became more widespread in the 1990s following implementation of monitoring protocols. Earlier literature was limited in relevance and scope.

Selection of studies: our search query resulted in over 250 relevant manuscripts discussing CIN. We excluded studies that lacked direct clinical applicability, including those that did not involve patient data, review articles lacking original findings, and preclinical studies limited to animal or in vivo models. ‘Direct clinical evidence’ was defined as research involving patient populations or clinically relevant outcomes related to CIN. Studies were included if they addressed genetic influences for CIN, epidemiological factors impacting CIN, and monitoring guidelines for CIN.

Outcome Measures: The main focus was to compile a detailed review discussing potential mechanisms for the development of CIN with an emphasis on genetic factors, and how CIN can be monitored and managed. This approach sought to elucidate existing knowledge and identify gaps in research concerning CIN.

8. Conclusions

Clozapine remains a cornerstone in the management of treatment-resistant schizophrenia, yet its use is blunted by its diverse adverse effect profile and its reputation for causing CIN. Although the precise mechanisms of how clozapine induces neutropenia remain unknown, popular theories include direct cytotoxicity, immune response, and genetic predisposition. Among the various proposed mechanisms, the formation of toxic nitrenium metabolites and genetic predisposition, specifically involving HLA and SLCO gene variants, represent the two most substantiated pillars supporting current hypotheses. Increasing evidence supports a genetic basis for this adverse effect, particularly involving HLA-DQB1, HLA-B, SLCO1B3 and SLCO1B7, and ACKR1. Currently, monitoring the absolute neutrophil count of patients undergoing clozapine therapy is the best tool clinicians have to prevent the development of worsening neutropenia or even agranulocytosis. Continued investigation into the genetic and immunologic mechanisms of CIN may enable the development of risk-stratified treatment protocols and more refined monitoring guidelines. Ultimately, integration of pharmacogenomics into clinical decision-making has the potential to improve safety, reduce treatment hesitancy, and expand access to this uniquely effective therapy. However, despite its promise, the routine clinical use of pharmacogenomics remains limited due to suboptimal sensitivity, high costs, and variability across populations. Advancements in genetic sequencing technologies, broader population-based validation, and reductions in testing costs will be critical to realizing its full potential in clinical practice.