Dissecting hair breakage in alopecia areata: the central role of dysregulated cysteine homeostasis.

In the initial stages of Alopecia Areata (AA), the predominance of hair breakage or exclamation mark hairs serves as vital indicators of disease activity. These signs are non-invasive and are commonly employed in dermatoscopic examinations. Despite their clinical salience, the underlying etiology precipitating this hair breakage remains largely uncharted territory. Our exhaustive review of the existing literature points to a pivotal role for cysteine-a key amino acid central to hair growth-in these mechanisms. This review will probe and deliberate upon the implications of aberrant cysteine metabolism in the pathogenesis of AA. It will examine the potential intersections of cysteine metabolism with autophagy, ferroptosis, immunity, and psychiatric manifestations associated with AA. Such exploration could illuminate new facets of the disease's pathophysiology, potentially paving the way for innovative therapeutic strategies.

Pathogenesis of Alopecia Areata and Vitiligo: Commonalities and Differences.

Both alopecia areata (AA) and vitiligo are distinct, heterogenous, and complex disease entities, characterized by nonscarring scalp terminal hair loss and skin pigment loss, respectively. In AA, inflammatory cell infiltrates are in the deep reticular dermis close to the hair bulb (swarm of bees), whereas in vitiligo the inflammatory infiltrates are in the epidermis and papillary dermis. Immune privilege collapse has been extensively investigated in AA pathogenesis, including the suppression of immunomodulatory factors (e.g., transforming growth factor-ß (TGF-ß), programmed death-ligand 1 (PDL1), interleukin-10 (IL-10), α-melanocyte-stimulating hormone (α-MSH), and macrophage migration inhibitory factor (MIF)) and enhanced expression of the major histocompatibility complex (MHC) throughout hair follicles. However, immune privilege collapse in vitiligo remains less explored. Both AA and vitiligo are autoimmune diseases that share commonalities in pathogenesis, including the involvement of plasmacytoid dendritic cells (and interferon-α (IFN- α) signaling pathways) and cytotoxic CD8+ T lymphocytes (and activated IFN-γ signaling pathways). Blood chemokine C-X-C motif ligand 9 (CXCL9) and CXCL10 are elevated in both diseases. Common factors that contribute to AA and vitiligo include oxidative stress, autophagy, type 2 cytokines, and the Wnt/ß-catenin pathway (e.g., dickkopf 1 (DKK1)). Here, we summarize the commonalities and differences between AA and vitiligo, focusing on their pathogenesis.

MiR-200c-3p as a novel genetic marker and therapeutic tool for alopecia areata.

BACKGROUND: MicroRNAs (miRNAs) are small RNA molecules that regulate gene expression in diverse biological processes. They hold promise as therapeutic candidates for targeting human disease pathways, although our understanding of their gene regulatory mechanism remains incomplete. Alopecia areata (AA) is a prevalent inflammatory ailment distinguished by the infiltration of T cells targeting the anagen-stage hair follicles. The scarcity of effective remedies for AA may stem from limited understanding regarding its precise cellular mechanism. AIM: To investigate and examine the importance and role of the miR-200c-3p as a genetic indicator for AA, and its possible impact on disease progression. SUBJECTS AND METHODS: Case-control study included 65 patients with AA and 65 matched healthy controls. A real-time PCR technique was used to measure the expression of miR-200c-3p for both groups. Bioinformatic tools were used for prediction with genes and gene-gene interaction, and protein-protein interaction. RESULTS: The expression levels of miR-200c-3p were significantly higher in AA patients than in healthy controls. We predicted that miR-200c-3p plays a markable role in the development of AA by its effect on the EGFR tyrosine kinase inhibitor resistance pathway. CONCLUSION: We were able to identify the influence of miR-200c-3p on both PLCG1 and RPS6KP1 genes which in turn regulate the EGFR tyrosine kinases resistance pathway that displayed the most substantial increase in activity. Our outcomes shed light on the era of the potential theranostic role of this innovative miRNA in AA.

Identification of SLC40A1, LCN2, CREB5, and SLC7A11 as ferroptosis-related biomarkers in alopecia areata through machine learning.

Alopecia areata (AA) is a common non-scarring hair loss condition driven by the collapse of immune privilege and oxidative stress. The role of ferroptosis, a type of cell death linked to oxidative stress, in AA is yet to be explored, even though it's implicated in various diseases. Using transcriptome data from AA patients and controls from datasets GSE68801 and GSE80342, we aimed to identify AA diagnostic marker genes linked to ferroptosis. We employed Single-sample gene set enrichment analysis (ssGSEA) for immune cell infiltration evaluation. Correlations between ferroptosis-related differentially expressed genes (FRDEGs) and immune cells/functions were identified using Spearman analysis. Feature selection was done through Support vector machine-recursive feature elimination (SVM-RFE) and LASSO regression models. Validation was performed using the GSE80342 dataset, followed by hierarchical internal validation. We also constructed a nomogram to assess the predictive ability of FRDEGs in AA. Furthermore, the expression and distribution of these molecules were confirmed through immunofluorescence. Four genes, namely SLC40A1, LCN2, CREB5, and SLC7A11, were identified as markers for AA. A prediction model based on these genes showed high accuracy (AUC = 0.9052). Immunofluorescence revealed reduced expression of these molecules in AA patients compared to normal controls (NC), with SLC40A1 and CREB5 showing significant differences. Notably, they were primarily localized to the outer root sheath and in proximity to the sebaceous glands. Our study identified several ferroptosis-related genes associated with AA. These findings, emerging from the integration of immune cell infiltration analysis and machine learning, contribute to the evolving understanding of diagnostic and therapeutic strategies in AA. Importantly, this research lays a solid foundation for subsequent studies exploring the intricate relationship between AA and ferroptosis.

Association of Obesity and Bariatric Surgery on Hair Health.

Obesity and obesity-related conditions today constitute a public health problem worldwide. Obesity is an "epidemic" chronic disorder, which is defined by the WHO as normal or excessive fat accumulation that may impair health. It is also defined for adults as a BMI that is greater than or equal to 30. The most common obesity-related diseases are type 2 diabetes mellitus, cardiovascular diseases, metabolic syndrome, chronic kidney disease, hyperlipidemia, hypertension, nonalcoholic fatty liver disease, and certain types of cancer. It has been also proven that obesity can have a negative effect on hair. It can lead to hair thinning. Patients with obesity can undergo bariatric surgery if they meet the inclusion criteria. The four common types of weight loss surgery include a duodenal switch with biliopancreatic diversion, laparoscopic adjustable gastric banding, Roux-en-Y gastric bypass, and sleeve gastrectomy. Bariatric surgery can affect skin and hair and is associated with telogen effluvium due to weight loss, microelement deficiency, anesthesia, low calorie intake, and low protein intake. Patients who undergo bariatric surgery can experience post-bariatric surgery depression. Hair loss can have a major impact on self-esteem, negatively affecting one's self-image. The purpose of this narrative review is to critically review how obesity, obesity-related diseases, and bariatric surgery affect hair health in general and the hair development cycle, and how they influence hair loss.

Properties of FDA-approved small molecule protein kinase inhibitors: A 2024 update.

Owing to the dysregulation of protein kinase activity in many diseases including cancer, this enzyme family has become one of the most important drug targets in the 21st century. There are 80 FDA-approved therapeutic agents that target about two dozen different protein kinases and seven of these drugs were approved in 2023. Of the approved drugs, thirteen target protein-serine/threonine protein kinases, four are directed against dual specificity protein kinases (MEK1/2), twenty block nonreceptor protein-tyrosine kinases, and 43 inhibit receptor protein-tyrosine kinases. The data indicate that 69 of these drugs are prescribed for the treatment of neoplasms. Six drugs (abrocitinib, baricitinib, deucravacitinib, ritlecitinib, tofacitinib, upadacitinib) are used for the treatment of inflammatory diseases (atopic dermatitis, rheumatoid arthritis, psoriasis, alopecia areata, and ulcerative colitis). Of the 80 approved drugs, nearly two dozen are used in the treatment of multiple diseases. The following seven drugs received FDA approval in 2023: capivasertib (HER2-positive breast cancer), fruquintinib (metastatic colorectal cancer), momelotinib (myelofibrosis), pirtobrutinib (mantle cell lymphoma, chronic lymphocytic leukemia, small lymphocytic lymphoma), quizartinib (Flt3-mutant acute myelogenous leukemia), repotrectinib (ROS1-positive lung cancer), and ritlecitinib (alopecia areata). All of the FDA-approved drugs are orally effective with the exception of netarsudil, temsirolimus, and trilaciclib. This review summarizes the physicochemical properties of all 80 FDA-approved small molecule protein kinase inhibitors including the molecular weight, number of hydrogen bond donors/acceptors, polar surface area, potency, solubility, lipophilic efficiency, and ligand efficiency.

Status of research on the development and regeneration of hair follicles.

Hair loss, or alopecia, is a prevalent condition in modern society that imposes substantial mental and psychological burden on individuals. The types of hair loss, include androgenetic alopecia, alopecia areata, and telogen effluvium; of them, androgenetic alopecia is the most common condition. Traditional treatment modalities mainly involve medical options, such as minoxidil, finasteride and surgical interventions, such as hair transplantation. However, these treatments still have many limitations. Therefore, exploring the pathogenesis of hair loss, specifically focusing on the development and regeneration of hair follicles (HFs), and developing new strategies for promoting hair regrowth are essential. Some emerging therapies for hair loss have gained prominence; these therapies include low-level laser therapy, micro needling, fractional radio frequency, platelet-rich plasma, and stem cell therapy. The aforementioned therapeutic strategies appear promising for hair loss management. In this review, we investigated the mechanisms underlying HF development and regeneration. For this, we studied the structure, development, cycle, and cellular function of HFs. In addition, we analyzed the symptoms, types, and causes of hair loss as well as its current conventional treatments. Our study provides an overview of the most effective regenerative medicine-based therapies for hair loss.

Integrated Safety Analysis of Ritlecitinib, an Oral JAK3/TEC Family Kinase Inhibitor, for the Treatment of Alopecia Areata from the ALLEGRO Clinical Trial Program.

BACKGROUND: The ALLEGRO phase 2a and 2b/3 studies demonstrated that ritlecitinib, an oral JAK3/TEC family kinase inhibitor, is efficacious at doses of ≥ 30 mg in patients aged ≥ 12 years with alopecia areata (AA). OBJECTIVE: The objective of this study was to evaluate the safety of ritlecitinib in an integrated analysis of four studies in AA. METHODS: Two cohorts were analyzed: a placebo-controlled and an all-exposure cohort. Proportions and study size-adjusted incidence rates (IRs) of adverse events (AEs) of interest and laboratory abnormalities are reported. RESULTS: In the placebo-controlled cohort (n = 881; median exposure: 169 days), the proportion of ritlecitinib-treated patients with AEs was 70.2-75.4% across doses versus 69.5% in the placebo group; serious AEs occurred in 0-3.2% versus 1.9% for the placebo. A total of 19 patients permanently discontinued due to AEs (5 while receiving the placebo). In the all-exposure cohort (n = 1294), median ritlecitinib exposure was 624 days [2091.7 total patient-years (PY)]. AEs were reported in 1094 patients (84.5%) and serious AEs in 57 (4.4%); 78 (6.0%) permanently discontinued due to AEs. The most common AEs were headache (17.7%; 11.9/100 PY), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) positive test (15.5%; 9.8/100 PY), and nasopharyngitis (12.4%; 8.2/100 PY). There were two deaths (breast cancer and acute respiratory failure/cardiorespiratory arrest). Proportions (IRs) were < 0.1% (0.05/100 PY) for opportunistic infections, 1.5% (0.9/100 PY) for herpes zoster, 0.5% (0.3/100 PY) for malignancies (excluding nonmelanoma skin cancer), and 0.2% (0.1/100 PY) for major adverse cardiovascular events. CONCLUSIONS: Ritlecitinib is well tolerated with an acceptable safety profile up to 24 months in patients aged ≥ 12 years with AA (video abstract and graphical plain language summary available). TRIAL REGISTRIES: ClinicalTrials.gov: NCT02974868 (date of registration: 11/29/2016), NCT04517864 (08/18/2020), NCT03732807 (11/07/2018), and NCT04006457 (07/05/2019).

Treatment preferences of adults and adolescents with alopecia areata: A discrete choice experiment.

PRODUCTS with janus kinase (JAK) inhibition have been shown to promote hair regrowth in patients with alopecia areata (AA). To guide drug-approval and treatment decisions, it is important to understand patients' willingness to accept the potential risks of JAK inhibition in exchange for potential benefits. We quantified the treatment preferences of adult (≥18 years) and adolescent patients (12-17 years) with AA in the US and Europe to determine the trade-offs they are willing to make between benefits and risks. Preferences for oral AA treatment attributes were elicited using a discrete choice experiment consisting of 12 tasks in which patients chose between two hypothetical treatment alternatives and no treatment. Benefits included the probability of 80%-100% scalp hair regrowth (Severity of Alopecia Tool score ≤ 20) and achieving moderate-to-normal eyebrow and eyelash hair. Treatment-related risks included 3-year probabilities of serious infection, cancer, and blood clots. Preference estimates were used to calculate the maximum level of each risk that patients were willing to accept for increases in treatment benefits. The most important attribute to both adults (n = 201) and adolescents (n = 120) was a 50% probability of achieving hair regrowth on most or all the scalp; however, adolescents placed greater relative importance on this attribute than did adults. Adults were averse to the risks of serious infection, cancer, and blood clots, whereas adolescents were averse to the risk of cancer. For a 20% increase in the probability of 80%-100% scalp hair regrowth, adults were willing to accept a mean (95% confidence interval) 3-year risk of serious infection, cancer, and blood clots of 7.4% (5.5-9.3), 2.5% (1.9-3.1), and 9.3% (6.4-12.2). Adolescents were willing to accept a 3-year risk of cancer of 3.3% (2.4-4.2). Patients with AA in the US and Europe are willing to accept substantial risks to obtain an effective treatment.

The crosstalk between PTEN-induced kinase 1-mediated mitophagy and the inflammasome in the pathogenesis of alopecia areata.

Alopecia areata (AA) is a T-cell-mediated autoimmune disease that causes chronic, relapsing hair loss; however, its precise pathogenesis remains to be elucidated. Recent studies have provided compelling evidence of crosstalk between inflammasomes and mitophagy-a process that contributes to the removal of damaged mitochondria. Our previous studies showed that the NLR family pyrin domain containing 3 (NLRP3) inflammasome is important for eliciting and progressing inflammation in AA. In this study, we detected mitochondrial DNA damage in AA-affected scalp tissues and IFNγ and poly(I:C) treated outer root sheath (ORS) cells. In addition, IFNγ and poly(I:C) treatment increased mitochondrial reactive oxygen species (ROS) levels in ORS cells. Moreover, we showed that mitophagy induction alleviates IFNγ and poly(I:C)-induced NLRP3 inflammasome activation in ORS cells. Lastly, PTEN-induced kinase 1 (PINK1) knockdown increased NLRP3 inflammasome activation, indicating that PINK1-mediated mitophagy plays a critical role in NLRP3 inflammasome activation in ORS cells. This study supports previous studies showing that oxidative stress disrupts immune privilege status and promotes autoimmunity in AA. The results emphasize the significance of crosstalk between mitophagy and inflammasomes in the pathogenesis of AA. Finally, mitophagy factors regulating mitochondrial dysfunction and inhibiting inflammasome activation could be novel therapeutic targets for AA.

Exosomes derived from uMSCs promote hair regrowth in alopecia areata through accelerating human hair follicular keratinocyte proliferation and migration.

Alopecia areata (AA) is a complex genetic disease that results in hair loss due to an autoimmune-mediated attack on the hair follicle. Mesenchymal stem cells (MSCs) have great potential to induce hair regeneration due to their strong secretion ability and multidirectional differentiation. Recent studies have revealed that the therapeutic potential of MSCs comes from their secretion ability, which can produce large amounts of bioactive substances and regulate the key physiological functions of subjects. The secretion products of MSCs, such as vesicles, exosomes, and conditioned media, have significant advantages in preparing of biological products derived from stem cells. Human umbilical cord mesenchymal stem cells (uMSCs) are the best choice for exosome production. uMSCs are obtained from the human umbilical cord. The umbilical cord is easy to obtain, and the efficiency of uMSCs isolation and culture higher than that of obtaining MSCs from bone marrow or adipose tissue. In this study, we investigated the effects of exosomes released from uMSCs in AA mice. In summary, due to easy isolation and cultivation, simple preparation, and convenient storage, it is possible to obtain uMSCs, or uMSCs exosomes for research and clinical treatment.

Chronic diffuse alopecia in women: a retrospective review of histopathologic diagnoses.

INTRODUCTION: The diseases causing chronic diffuse alopecia and having similar clinical findings, namely chronic telogen effluvium, androgenetic alopecia, and the alopecia with overlapping features, should be differentiated. Recently, diffuse variants of lichen planopilaris have been described with histopathologic features of lichen planopilaris but clinically presenting with diffuse hair loss mostly in an androgenetic pattern. OBJECTIVES: To determine the accurate diagnosis underlying chronic diffuse alopecia in women by evaluating histopathologic findings. PATIENTS AND METHODS: The study included 32 patients with diffuse and clinically noncicatricial alopecia for at least 6 months with no identifiable etiologic factor after general medical history, review of organ systems, and appropriate laboratory tests. Two 4 mm punch biopsies, one from vertex and the other from mid-occiput, were obtained and sectioned transversely. RESULTS: The median age was 30.5 years (range: 22-40 years), and the median duration of hair loss was 4 years (range: 1.5-10 years). The histopathologic diagnosis was androgenetic alopecia, chronic telogen effluvium, and overlapping alopecia in 13 (40.6%), three (9.4%), and four (12.5%) patients, respectively. In the remaining 12 (32.5%) patients, a lichenoid inflammatory reaction affecting the infundibulum and isthmus was noted, and the probable diagnosis of diffuse variant of lichen planopilaris was made. LIMITATIONS: The retrospective nature and the small sample size. CONCLUSION: When the clinical diagnosis is not straightforward and no etiologic factor is found, histopathologic examination is mandatory for the accurate diagnosis of the disorder leading to chronic diffuse alopecia in women.

Potential Indications of Dupilumab in Th-2 Inflammatory Disease.

Dupilumab is a fully humanized IgG4 monoclonal antibody, inhibiting IL-4 and IL-13 signaling, which are the main cytokines involved in type 2 inflammatory diseases. Its introduction was a breakthrough in the treatment of moderate-to-severe atopic dermatitis, but it is also used in other inflammatory diseases, including asthma, eosinophilic esophagitis and chronic rhinosinusitis with nasal polyposis. Recent advances in the understanding of inflammatory pathways have revealed that Th2-type inflammation is involved in a wider range of diseases than previously thought. The aim of our review is to examine off-label therapeutic indications of dupilumab, including bullous dermatoses (pemphigus, bullous pemphigoid) and alopecia areata, and to investigate its potential applications in cancer patients on anti-PD1 therapy.

International Dermatology Outcome Measures (IDEOM) - Report from the 2022 Annual Meeting.

BACKGROUND: The International Dermatology Outcome Measures (IDEOM) is a non-profit organization dedicated to the advancement of evidence-based, consensus-driven outcome measures in dermatological diseases. Researchers and stakeholders from various backgrounds collaborate to develop these objective benchmark metrics to further advance treatment and management of dermatological conditions. SUMMARY: The 2022 IDEOM Annual Meeting was held on June 17-18, 2022. Leaders and stakeholders from the hidradenitis suppurativa, acne, vitiligo, actinic keratosis, alopecia areata, itch, cutaneous lymphoma, and psoriatic disease workgroups discussed the progress of their respective outcome-measures research. This report summarizes each workgroup's updates from 2022 and their next steps as established during the 2022 IDEOM Annual Meeting. J Drugs Dermatol. 2023;22(12):1153-1159 doi:10.36849/JDD.7615.

LITFULOTM (Ritlecitinib) Capsules: A Janus Kinase 3 Inhibitor for the Treatment of Severe Alopecia Areata.

LITFULOTM (ritlecitinib) capsules were recently approved for the treatment of severe alopecia areata in adolescents and adults, aged ≥12 years. Ritlecitinib is the active ingredient and a dual inhibitor of Janus kinase 3 and the tyrosine kinase expressed in hepatocellular carcinoma kinase family. It prevents immune attack on the hair follicles that leads to hair loss. In a phase 2b-3 dose-dependant study, five doses of oral ritlecitinib and placebo administered once daily (QD) were investigated. Ritlecitinib demonstrated efficacy in achieving the primary outcome, Severity of Alopecia Tool (SALT) score of ≤20, at week 24 (31% [38/124] 200-mg ritlecitinib QD for 4 weeks, then 50 mg QD for 20 weeks; 22% [27/121] 200-mg ritlecitinib QD for 4 weeks, then 30 mg QD for 20 weeks; 23% [29/124] 50-mg ritlecitinib QD; 14% [17/119] 30-mg ritlecitinib QD; 2% [1/59] 10-mg ritlecitinib QD; and 2% [2/130] placebo). Mild to moderate common adverse effects were observed, which included headache, nasopharyngitis, and upper respiratory tract infection. The recommended regimen of ritlecitinib capsules is 50 mg QD with without food and swallowed whole.

Inhibition of T-cell activity in alopecia areata: recent developments and new directions.

Alopecia areata (AA) is an autoimmune disease that has a complex underlying immunopathogenesis characterized by nonscarring hair loss ranging from small bald patches to complete loss of scalp, face, and/or body hair. Although the etiopathogenesis of AA has not yet been fully characterized, immune privilege collapse at the hair follicle (HF) followed by T-cell receptor recognition of exposed HF autoantigens by autoreactive cytotoxic CD8+ T cells is now understood to play a central role. Few treatment options are available, with the Janus kinase (JAK) 1/2 inhibitor baricitinib (2022) and the selective JAK3/tyrosine kinase expressed in hepatocellular carcinoma (TEC) inhibitor ritlecitinib (2023) being the only US Food and Drug Administration-approved systemic medications thus far for severe AA. Several other treatments are used off-label with limited efficacy and/or suboptimal safety and tolerability. With an increased understanding of the T-cell-mediated autoimmune and inflammatory pathogenesis of AA, additional therapeutic pathways beyond JAK inhibition are currently under investigation for the development of AA therapies. This narrative review presents a detailed overview about the role of T cells and T-cell-signaling pathways in the pathogenesis of AA, with a focus on those pathways targeted by drugs in clinical development for the treatment of AA. A detailed summary of new drugs targeting these pathways with expert commentary on future directions for AA drug development and the importance of targeting multiple T-cell-signaling pathways is also provided in this review.

Linear alopecia in pediatrics: RCM and dermoscopy reveal diagnostic cues.

BACKGROUND: Alopecia areata (AA), trichotillomania (TM), nevus sebaceous (NS), and linear scleroderma en coup de sabre (LSCS) can all present with a patch of linear alopecia, making diagnosis challenging. The purpose of this study was to combine reflectance confocal microscopy (RCM) and dermoscopy in the diagnosis of these lesions in children. METHODS: A total of 36 patients with linear alopecia were enrolled, of whom 14 had AA, seven had TM, nine had NS, and six had LSCS. We evaluated the characteristics and distinguishing features of the four conditions using RCM and dermoscopy. RESULTS: The key to differential diagnosis was the dermal Hair follicle density in the dermis was decreased in AA, and the size and density of the follicular openings were normal in TM. In NS, the major features were petal-like and frogspawn-like structures. In LSCS, dermal papillary rings, sebaceous glands, and follicles were partially or completely missing, and abundant fibrous material was distributed in the dermis. Dermoscopy revealed alopecia, and all four conditions resulted in decreased hair density. AA patients exhibited yellow dots, black dots, and exclamation mark hairs. TM patients presented with irregularly broken hairs and blood spots. Both NS and LSCS patients exhibited an absence of follicular openings; NS patients demonstrated whitish and yellowish round structures, while an atrophic area with white patches, linear vessels, and no yellow or black dots was observed in LSCS patients CONCLUSION: RCM combined with dermoscopy can provide additional information on disease states and differentiate between AA, TM, NS, and LSCS.

Evolution of Ritlecitinib Population Pharmacokinetic Models During Clinical Drug Development.

BACKGROUND: Ritlecitinib is an oral Janus kinase 3/tyrosine kinase expressed in hepatocellular carcinoma family inhibitor undergoing parallel clinical development for alopecia areata, vitiligo, ulcerative colitis, Crohn's disease, and rheumatoid arthritis. OBJECTIVE: As studies read out simultaneously, strategic planning of population pharmacokinetic model development and evaluation is required to ensure timely decisions. METHODS: Data from healthy participants and patients from 12 clinical trials between December 2014 and July 2021 were included: seven phase I studies in healthy participants and organ impairment, five phase II/III studies in patients with rheumatoid arthritis, ulcerative colitis, alopecia areata, and vitiligo. Population pharmacokinetic models consisted of stepwise procedures to accommodate data availability and the model's application to answering clinical development questions. At each iteration of the model update, parameters of the next model were re-estimated by leveraging previous information and new data. RESULTS: Three model development lifecycle iterations of the ritlecitinib population pharmacokinetic model were conducted to support alopecia areata, vitiligo, and ulcerative colitis study readouts. Initial structural modeling based on healthy participant data (and some rheumatoid arthritis and alopecia areata data) in iteration 1 provided a platform for comprehensive covariate testing during iteration 2, and model evaluation and implementation of the frequentist prior approach in iteration 3. The final model was a two-compartment model with first-order absorption and direct-response non-stationary clearance and bioavailability driven by concentrations in the peripheral compartment. CONCLUSIONS: The present approach demonstrated the evolution of three population pharmacokinetic models with accumulating data, addressed clinical drug development questions related to systemic exposures of ritlecitinib, and informed the approved product label. CLINICAL TRIAL REGISTRATION: NCT02309827, NCT02684760, NCT02958865, NCT02969044, NCT03232905, NCT03732807, NCT04016077, NCT03715829, NCT04037865, NCT04004663, NCT04634565, NCT02974868.