AbbVie (Japan)

Human melanocytes are distributed not only in the epidermis and in hair follicles but also in mucosa, cochlea (ear), iris (eye), and mesencephalon (brain) among other tissues. Melanocytes, which are derived from the neural crest, are unique in that they produce eu-/pheo-melanin pigments in unique membrane-bound organelles termed melanosomes, which can be divided into four stages depending on their degree of maturation. Pigmentation production is determined by three distinct elements: enzymes involved in melanin synthesis, proteins required for melanosome structure, and proteins required for their trafficking and distribution. Many genes are involved in regulating pigmentation at various levels, and mutations in many of them cause pigmentary disorders, which can be classified into three types: hyperpigmentation (including melasma), hypopigmentation (including oculocutaneous albinism [OCA]), and mixed hyper-/hypopigmentation (including dyschromatosis symmetrica hereditaria). We briefly review vitiligo as a representative of an acquired hypopigmentation disorder.

Gastric cancer (GC) is a highly heterogeneous disease. To identify potential clinically actionable therapeutic targets that may inform individualized treatment strategies, we performed whole-exome sequencing on 78 GCs of differing histologies and anatomic locations, as well as whole-genome sequencing on two GC cases, each with three primary tumors and two matching lymph node metastases. The data showed two distinct GC subtypes with either high-clonality (HiC) or low-clonality (LoC). The HiC subtype of intratumoral heterogeneity was associated with older age, TP53 (tumor protein P53) mutation, enriched C > G transition, and significantly shorter survival, whereas the LoC subtype was associated with younger age, ARID1A (AT rich interactive domain 1A) mutation, and significantly longer survival. Phylogenetic tree analysis of whole-genome sequencing data from multiple samples of two patients supported the clonal evolution of GC metastasis and revealed the accumulation of genetic defects that necessitate combination therapeutics. The most recurrently mutated genes, which were validated in a separate cohort of 216 cases by targeted sequencing, were members of the homologous recombination DNA repair, Wnt, and PI3K-ERBB pathways. Notably, the drugable NRG1 (neuregulin-1) and ERBB4 (V-Erb-B2 avian erythroblastic leukemia viral oncogene homolog 4) ligand-receptor pair were mutated in 10% of GC cases. Mutations of the BRCA2 (breast cancer 2, early onset) gene, found in 8% of our cohort and validated in The Cancer Genome Atlas GC cohort, were associated with significantly longer survivals. These data define distinct clinicogenetic forms of GC in the Chinese population that are characterized by specific mutation sets that can be investigated for efficacy of single and combination therapies.

OBJECTIVES: To confirm the safety and effectiveness of adalimumab and to evaluate the influence of the concomitant use of methotrexate (MTX). METHODS: Postmarketing surveillance of 7740 Japanese rheumatoid arthritis (RA) patients was performed. All patients who received adalimumab in the registration period were followed for 28 weeks after starting treatment for safety and 24 weeks for effectiveness. Effectiveness was measured by duration of morning stiffness, swollen and tender joint counts (28 joints), patient global assessment of disease activity, erythrocyte sedimentation rate and serum C-reactive protein. RESULTS: Comparable rates of adverse drug reactions (ADRs) were reported in this study and in the interim analysis. Age, pulmonary disease history or comorbidity, co-existing diabetes mellitus, concomitant MTX at doses of > 8 mg/week and concomitant glucocorticoids at doses of > 5 mg/day were risk factors for infections. All mean values of effectiveness measurements improved. Relatively lower disease activity at baseline, biologic-naïve, concomitant MTX use and early RA stage/low functional class were background factors contributing to the effectiveness. The combination of adalimumab with MTX improved the response to adalimumab treatment. CONCLUSION: Adalimumab, especially with concomitant use of MTX, provided significant improvement in disease activity, without any unexpected ADRs in Japanese RA patients.

Safety related drug failures continue to be a challenge for pharmaceutical companies despite the numerous complex and lengthy in vitro assays and in vivo studies that make up the typical safety screening funnel. A lack of complete translation of animal data to humans can explain some of those shortcomings. Differences in sensitivity and drug disposition between animals and humans may also play a role. Many gaps exist for potential target tissues of drugs that cannot be adequately modeled in vitro. Microphysiological systems (MPS) may help to better model these target tissues and provide an opportunity to better assess some aspects of human safety prior to clinical studies. There is hope that these systems can supplement current preclinical drug safety and disposition evaluations, filling gaps and enhancing our ability to predict and understand human relevant toxicities. The International Consortium for Innovation and Quality in Pharmaceutical Development (IQ) MPS Affiliate is a group of pharmaceutical industry scientists who seek to expedite appropriate characterization and incorporation of MPS to potentially improve drug safety assessment and provide safer and more effective medicines to patients. In keeping with this mission, the IQ MPS Affiliate scientists have prepared a series of organotypic manuscripts for several key drug safety and disposition target tissues (lung, liver, kidney, skin, gastrointestinal, cardiovascular, and blood brain barrier/central nervous system). The goal of these manuscripts is to provide key information related to likely initial contexts of use (CoU) and key characterization data needed for incorporation of MPS in pharmaceutical safety screening including a list of characteristic functions, cell types, toxicities, and test agents (representing major mechanisms of toxicity) that can be used by MPS developers. Additional manuscripts focusing on testing biologically based therapeutics and ADME considerations have been prepared as part of this effort. These manuscripts focus on general needs for assessing biologics and ADME endpoints and include similar information to the tissue specific manuscripts where appropriate. The current manuscript is an introduction to several general concepts related to pharmaceutical industry needs with regard to MPS application and other MPS concepts that apply across the organ specific manuscripts.

Histopathologic examination of the testis is the most sensitive means to detect effects on spermatogenesis; however, the complexity of testicular histology, interrelatedness of cell types within the testis, and long duration of spermatogenesis can make assessment of a testicular toxicant challenging. A thorough understanding of the histology and morphologic manifestations of response to injury is critical to successfully identify a testicular effect and to begin to understand the underlying mechanism of action. The basic patterns of response to xenobiotic-induced injury to the testis and epididymis are detailed and discussed.

BACKGROUND: We aimed to characterize the natural history of hepatitis C virus (HCV) reinfection and spontaneous clearance following reinfection (reclearance), including predictors of HCV reclearance. METHODS: Data were synthesized from the 9 prospective cohorts of the International Collaboration of Incident Human Immunodeficiency Virus and HCV in Injecting Cohorts study, which evaluated HCV infection outcomes among people who inject drugs. Participants with primary HCV infection were classified as having achieved viral suppression if they had negative results of at least 1 subsequent HCV RNA test. Those with positive results of an HCV RNA test following viral suppression were investigated for reinfection. Viral sequence analysis was used to identify reinfection (defined as detection of heterologous virus with no subsequent detection of the original viral strain). RESULTS: Among 591 participants with acute primary HCV infection, 118 were investigated for reinfection. Twenty-eight participants were reinfected (12.3 cases/100 person-years; 95% confidence interval [CI], 8.5-17.8). Peak HCV RNA level was lower during reinfection than primary infection (P = .011). The proportion of individuals with reclearance 6 months after reinfection was 52% (95% CI, 33%-73%). After adjustment for study site, females with the IFNL4 (formerly IFNL3 and IL28B) rs12979860 CC genotype detected were more likely to have reclearance (hazard ratio, 4.16; 95% CI, 1.24-13.94; P = .021). CONCLUSIONS: Sex and IFNL4 genotype are associated with spontaneous clearance after reinfection.

Tumor immunology has changed the landscape of cancer treatment. Yet, not all patients benefit as cancer immune responsiveness (CIR) remains a limitation in a considerable proportion of cases. The multifactorial determinants of CIR include the genetic makeup of the patient, the genomic instability central to cancer development, the evolutionary emergence of cancer phenotypes under the influence of immune editing, and external modifiers such as demographics, environment, treatment potency, co-morbidities and cancer-independent alterations including immune homeostasis and polymorphisms in the major and minor histocompatibility molecules, cytokines, and chemokines. Based on the premise that cancer is fundamentally a disorder of the genes arising within a cell biologic process, whose deviations from normality determine the rules of engagement with the host's response, the Society for Immunotherapy of Cancer (SITC) convened a task force of experts from various disciplines including, immunology, oncology, biophysics, structural biology, molecular and cellular biology, genetics, and bioinformatics to address the complexity of CIR from a holistic view. The task force was launched by a workshop held in San Francisco on May 14-15, 2018 aimed at two preeminent goals: 1) to identify the fundamental questions related to CIR and 2) to create an interactive community of experts that could guide scientific and research priorities by forming a logical progression supported by multiple perspectives to uncover mechanisms of CIR. This workshop was a first step toward a second meeting where the focus would be to address the actionability of some of the questions identified by working groups. In this event, five working groups aimed at defining a path to test hypotheses according to their relevance to human cancer and identifying experimental models closest to human biology, which include: 1) Germline-Genetic, 2) Somatic-Genetic and 3) Genomic-Transcriptional contributions to CIR, 4) Determinant(s) of Immunogenic Cell Death that modulate CIR, and 5) Experimental Models that best represent CIR and its conversion to an immune responsive state. This manuscript summarizes the contributions from each group and should be considered as a first milestone in the path toward a more contemporary understanding of CIR. We appreciate that this effort is far from comprehensive and that other relevant aspects related to CIR such as the microbiome, the individual's recombined T cell and B cell receptors, and the metabolic status of cancer and immune cells were not fully included. These and other important factors will be included in future activities of the taskforce. The taskforce will focus on prioritization and specific actionable approach to answer the identified questions and implementing the collaborations in the follow-up workshop, which will be held in Houston on September 4-5, 2019.

OBJECTIVES: Little biomechanical information is available about kinematically aligned (KA) total knee arthroplasty (TKA). The purpose of this study was to simulate the kinematics and kinetics after KA TKA and mechanically aligned (MA) TKA with four different limb alignments. MATERIALS AND METHODS: Bone models were constructed from one volunteer (normal) and three patients with three different knee deformities (slight, moderate and severe varus). A dynamic musculoskeletal modelling system was used to analyse the kinematics and the tibiofemoral contact force. The contact stress on the tibial insert, and the stress to the resection surface and medial tibial cortex were examined by using finite element analysis. RESULTS: In all bone models, posterior translation on the lateral side and external rotation in the KA TKA models were greater than in the MA TKA models. The tibiofemoral force at the medial side was increased in the moderate and severe varus models with KA TKA. In the severe varus model with KA TKA, the contact stress on the tibial insert and the stress to the resection surface and to the medial tibial cortex were increased by 41.5%, 32.2% and 53.7%, respectively, compared with MA TKA, and the bone strain at the medial side was highest among all models. CONCLUSION: Near normal kinematics was observed in KA TKA. However, KA TKA increased the contact force, stress and bone strain at the medial side for moderate and severe varus knee models. The application of KA TKA for severe varus knees may be inadequate.Cite this article: S. Nakamura, Y. Tian, Y. Tanaka, S. Kuriyama, H. Ito, M. Furu, S. Matsuda. The effects of kinematically aligned total knee arthroplasty on stress at the medial tibia: A case study for varus knee. Bone Joint Res 2017;6:43-51. DOI: 10.1302/2046-3758.61.BJR-2016-0090.R1.

BACKGROUND: Parkinson's disease is now one of the fastest-growing neurodegenerative disorders in the developed world, with an increasing prevalence and associated socioeconomic costs. Progression of the disease leads to a gradual deterioration in patients' quality of life, despite optimal treatment, and both medical and societal needs increase, often with the assistance of paid and/or unpaid caregivers. OBJECTIVE: We aimed to quantify the incremental economic burden of Parkinson's disease by disease severity in a real-world setting across differing geographic regions. METHODS: Demographics, clinical characteristics, health status, patient quality of life, caregiver burden, and healthcare resource utilization data were drawn from the Adelphi Parkinson's Disease Specific Program™, conducted in the USA, five European countries, and Japan. RESULTS: A total of 563 neurologists provided data for 5299 individuals with Parkinson's disease; 61% were male, with a mean age of 64 years. Approximately 15% of individuals were deemed to have advanced disease, with significantly more comorbidities, and a poorer quality of life, than those with non-advanced disease. Overall, the mean annual healthcare resource utilization increased significantly with advancing disease, and resulted in a three-fold difference in the USA and Europe. The main drivers behind the high economic burden included hospitalizations, prescription medications, and indirect costs. CONCLUSIONS: People with Parkinson's disease, and their caregivers, incur a higher economic burden as their disease progresses. Future interventions that can control symptoms or slow disease progression could reduce the burden on people with Parkinson's disease and their caregivers, whilst also substantially impacting societal costs.

Anti-cancer immunotherapy is encountering its own checkpoint. Responses are dramatic and long lasting but occur in a subset of tumors and are largely dependent upon the pre-existing immune contexture of individual cancers. Available data suggest that three landscapes best define the cancer microenvironment: immune-active, immune-deserted and immune-excluded. This trichotomy is observable across most solid tumors (although the frequency of each landscape varies depending on tumor tissue of origin) and is associated with cancer prognosis and response to checkpoint inhibitor therapy (CIT). Various gene signatures (e.g. Immunological Constant of Rejection - ICR and Tumor Inflammation Signature - TIS) that delineate these landscapes have been described by different groups. In an effort to explain the mechanisms of cancer immune responsiveness or resistance to CIT, several models have been proposed that are loosely associated with the three landscapes. Here, we propose a strategy to integrate compelling data from various paradigms into a "Theory of Everything". Founded upon this unified theory, we also propose the creation of a task force led by the Society for Immunotherapy of Cancer (SITC) aimed at systematically addressing salient questions relevant to cancer immune responsiveness and immune evasion. This multidisciplinary effort will encompass aspects of genetics, tumor cell biology, and immunology that are pertinent to the understanding of this multifaceted problem.

Drug-induced gastrointestinal toxicities (DI-GITs) are among the most common adverse events in clinical trials. High prevalence of DI-GIT has persisted among new drugs due in part to the lack of robust experimental tools to allow early detection or to guide optimization of safer molecules. Developing in vitro assays for the leading GI toxicities (nausea, vomiting, diarrhoea, constipation, and abdominal pain) will likely involve recapitulating complex physiological properties that require contributions from diverse cell/tissue types including epithelial, immune, microbiome, nerve, and muscle. While this stipulation may be beyond traditional 2D monocultures of intestinal cell lines, emerging 3D GI microtissues capture interactions between diverse cell and tissue types. These interactions give rise to microphysiologies fundamental to gut biology. For GI microtissues, organoid technology was the breakthrough that introduced intestinal stem cells with the capability of differentiating into each of the epithelial cell types and that self-organize into a multi-cellular tissue proxy with villus- and crypt-like domains. Recently, GI microtissues generated using miniaturized devices with microfluidic flow and cyclic peristaltic strain were shown to induce Caco2 cells to spontaneously differentiate into each of the principle intestinal epithelial cell types. Second generation models comprised of epithelial organoids or microtissues co-cultured with non-epithelial cell types can successfully reproduce cross-'tissue' functional interactions broadening the potential of these models to accurately study drug-induced toxicities. A new paradigm in which in vitro assays become an early part of GI safety assessment could be realized if microphysiological systems (MPS) are developed in alignment with drug-discovery needs. Herein, approaches for assessing GI toxicity of pharmaceuticals are reviewed and gaps are compared with capabilities of emerging GI microtissues (e.g., organoids, organ-on-a-chip, transwell systems) in order to provide perspective on the assay features needed for MPS models to be adopted for DI-GIT assessment.

BACKGROUND: This study assessed the efficacy and safety of briakinumab, a human anti-IL-12/23p40 monoclonal antibody, compared with placebo for the induction and maintenance of remission in patients with moderately to severely active Crohn's disease. METHODS: In this phase 2b, multicenter, double-blind, parallel group study, 246 patients stratified by prior tumor necrosis factor-antagonist use and response, were randomized (1:1:1:3) to 4 intravenous induction regimens: placebo, 200, 400, or 700 mg briakinumab, at weeks 0/4/8. At week 12, responders in the placebo or 400-mg induction groups entered the maintenance phase with the same regimen, whereas responders in the 700-mg induction group were rerandomized (1:1:1) to receive placebo, 200, or 700 mg briakinumab at weeks 12/16/20. At week 24, patients in remission stopped receiving study drug (withdrawal phase) until relapse. Patients experiencing relapse, nonresponders, and nonremitters could enter the open-label phase. RESULTS: The primary end point of clinical remission at week 6 was not met. There were numerically greater rates of remission and response at 6, 12, or 24 weeks in patients treated with briakinumab. The safety and tolerability profile of briakinumab was similar in the induction and maintenance phases of the trial. CONCLUSIONS: Briakinumab showed a similar safety and tolerability profile to placebo in the induction and maintenance phases, and comparable rates of serious adverse events, adverse events leading to discontinuation, and malignancy. These data provide support for the potential efficacy of briakinumab and other IL-12/23 inhibitors in the treatment of moderate-to-severe Crohn's disease.

BACKGROUND & AIMS: SERENE UC (Study of a Novel Approach to Induction and Maintenance Dosing With Adalimumab in Patients With Moderate to Severe Ulcerative Colitis) evaluated the efficacy of higher adalimumab induction and maintenance dose regimens in patients with ulcerative colitis. METHODS: This phase 3, double-blind, randomized trial included induction and maintenance studies, with a main study (ex-Japan) and Japan substudy. Eligible patients (18-75 years, full Mayo score 6-12, centrally read endoscopy subscore 2-3) were randomized 3:2 to higher induction regimen (adalimumab 160 mg at weeks 0, 1, 2, and 3) or standard induction regimen (160 mg at week 0 and 80 mg at week 2); all received 40 mg at weeks 4 and 6. At week 8, all patients were rerandomized 2:2:1 (main study) to 40 mg every week (ew), 40 mg every other week (eow), or exploratory therapeutic drug monitoring; or 1:1 (Japan substudy) to 40 mg ew or 40 mg eow maintenance regimens. RESULTS: In the main study, 13.3% vs 10.9% of patients receiving the higher induction regimen vs standard induction regimen achieved clinical remission (full Mayo score ≤2 with no subscore >1) at week 8 (induction primary end point; P = .265); among week-8 responders, 39.5% vs 29.0% receiving 40 mg ew vs 40 mg eow achieved clinical remission at week 52 (maintenance primary end point; P = .069). In the integrated (main + Japan) population, 41.1% vs 30.1% of week-8 responders receiving 40 mg ew vs 40 mg eow achieved clinical remission at week 52 (nominal P = .045). Safety profiles were comparable between dosing regimens. CONCLUSION: Although primary end points were not met, a >10% absolute difference in clinical remission was demonstrated with higher adalimumab maintenance dosing. Higher dosing regimens were generally well tolerated and consistent with the known safety profile of adalimumab in ulcerative colitis. CLINICALTRIALS: gov, Number: NCT002209456.

International audience

Defining responses of the structural and immune cells in biologic systems is critically important to understanding disease states and responses to injury. This requires accurate and sensitive methods to define cell types in organ systems. The principal method to delineate the cell populations involved in these processes is flow cytometry. Although researchers increasingly use flow cytometry, technical challenges can affect its accuracy and reproducibility, thus significantly limiting scientific advancements. This challenge is particularly critical to lung immunology, as the lung is readily accessible and therefore used in preclinical and clinical studies to define potential therapeutics. Given the importance of flow cytometry in pulmonary research, the American Thoracic Society convened a working group to highlight issues and technical challenges to the performance of high-quality pulmonary flow cytometry, with a goal of improving its quality and reproducibility.

We present two distinctly different posterior predictive approaches to Bayesian phylogenetic model selection and illustrate these methods using examples from green algal protein-coding cpDNA sequences and flowering plant rDNA sequences. The Gelfand-Ghosh (GG) approach allows dissection of an overall measure of model fit into components due to posterior predictive variance (GGp) and goodness-of-fit (GGg), which distinguishes this method from the posterior predictive P-value approach. The conditional predictive ordinate (CPO) method provides a site-specific measure of model fit useful for exploratory analyses and can be combined over sites yielding the log pseudomarginal likelihood (LPML) which is useful as an overall measure of model fit. CPO provides a useful cross-validation approach that is computationally efficient, requiring only a sample from the posterior distribution (no additional simulation is required). Both GG and CPO add new perspectives to Bayesian phylogenetic model selection based on the predictive abilities of models and complement the perspective provided by the marginal likelihood (including Bayes Factor comparisons) based solely on the fit of competing models to observed data.

OBJECTIVE: To evaluate upadacitinib efficacy and safety dose response in Japanese patients with active RA and an inadequate response to conventional synthetic DMARDs (csDMARDs). METHODS: This was a multicentre, phase IIb/III, dose-ranging study conducted in Japan, in which patients on previously stable csDMARDs were randomized to receive upadacitinib 7.5, 15 or 30 mg once daily or matching placebo for a 12-week double-blind period. The primary endpoint was a 20% improvement in ACR criteria (ACR20) response at week 12 using non-responder imputation. Key secondary endpoints included ACR50, ACR70 and 28-joint DAS with CRP (DAS28-CRP) remission and low disease activity. Adverse events were also assessed. RESULTS: Of 197 patients treated, 187 completed the double-blind period. At week 12, more patients receiving upadacitinib 7.5, 15 or 30 mg vs placebo met the ACR20 response (75.5%, 83.7%, 80.0% vs 42.9%; P < 0.001), with significant differences observed as early as week 1. Stringent responses, including ACR50, ACR70 and DAS28-CRP <2.6, were achieved by significantly higher proportions of patients on upadacitinib than placebo and by numerically higher proportions on upadacitinib 15 or 30 mg vs upadacitinib 7.5 mg. Adverse events and infections (serious infections, opportunistic infections and herpes zoster) were more common with upadacitinib vs placebo and numerically highest with upadacitinib 30 mg. There were no venous thromboembolic events reported. CONCLUSION: Efficacy of upadacitinib was demonstrated in this population of Japanese patients with RA and an inadequate response to csDMARDs. Safety and tolerability were consistent with other upadacitinib RA studies. The 15 mg dose of upadacitinib showed the most favourable benefit-risk profile. TRIAL REGISTRATION: ClinicalTrials.gov, https://clinicaltrials.gov/ct2/show/NCT02720523.

Humira® (adalimumab) is a recombinant human IgG1 monoclonal antibody (mAb) glycoprotein consisting of 1330 amino acids that is specific for human tumor necrosis factor (TNF). The biological activity and clinical profile of mAb therapeutics, including adalimumab, is influenced by their protein structure and glycosylation patterns, which can be affected by the expression system, cell culture conditions and purification process methodology. While clinical outcome cannot yet be attributed to many of the individual structural features that constitute a mAb, it is evident that detailed structural attribute analysis is necessary if structural contributions to function are to be comprehensively defined. Adalimumab product quality data generated from over a decade of manufacturing across multiple production sites and through a series of manufacturing scale changes are presented here. These data reveal a consistent and tightly controlled profile for the product.

BACKGROUND: Atopic dermatitis (AD), a relapsing, inflammatory skin disease, is associated with pruritus that can negatively affect patients' quality of life. Understanding the burden of AD is critical for informing and tailoring treatment and disease management to improve patient outcomes. This study characterized global treatment patterns and the clinical, psychosocial and economic burden of moderate-to-severe AD. METHODS: MEASURE-AD was a cross-sectional 28-country study in patients with physician-confirmed moderate-to-severe AD who were either receiving or eligible for systemic therapy for AD. Patients ≥12 years were enrolled between December 2019 and December 2020 while attending routine office or clinic visit. Primary outcomes included Worst Pruritus Numeric Rating Scale (WP-NRS; range: 0-10) and Dermatology Life Quality Index (DLQI; range: 0-30) and Children's DLQI (CDLQI; range: 0-30). Secondary outcomes included physician- and patient-reported clinical, psychosocial and economic burden. RESULTS: Of the 1591 patients enrolled, 1558 (1434 adults and 124 adolescents) fulfilled all patient selection criteria and were included in this analysis. Almost all patients (98.4%) in the total population were using AD medications and more than half (56%) were receiving systemic medication (15% systemic monotherapy). The most used systemic therapies were dupilumab (56.3%), systemic glucocorticoids (18.1%) and methotrexate (16.2%). Mean WP-NRS was 5.3 in the total population, and most patients (≥55%) reported moderate-to-severe pruritus (WP-NRS ≥4). Mean DLQI was 10.8 and mean CDLQI was 9.6. Secondary endpoints demonstrated substantial clinical, psychosocial, and economic burden of disease. Subgroup analysis demonstrated that patients receiving systemic therapy had lower disease burden than those not taking systemic medications. CONCLUSIONS: While systemic therapy lowers overall disease burden, patients with moderate-to-severe AD continue to have substantial multidimensional disease burden and uncontrolled disease. Overall, there is a need for effective disease management, including effective treatments that improve patients' psychosocial outcomes and reduce the economic burden of AD.

Longitudinal changes in a population of interest are often heterogeneous and influenced by a combination of baseline factors. In such cases, classical linear mixed effects models [Laird NM, Ware JH. Random-effects models for longitudinal data. Biometrics. 1982;38:963–974.] for the mean structure provide poor fit to the data. We propose regression tree methodology for the longitudinal data identifying and characterizing homogeneous subgroups. Currently available regression tree construction methods are either limited to a repeated measures scenario or combine the heterogeneity among subgroups with the random inter-subject variability. We propose a longitudinal classification and regression tree (LongCART) algorithm under conditional inference framework that overcomes these limitations utilizing a two-step approach. The LongCART first selects the partitioning variable via a parameter instability test and then finds the optimal split for the selected partitioning variable. Thus, at each node, the decision of further splitting is type I error controlled, guarding against variable selection bias, over-fitting and spurious splitting. We obtained asymptotic results for the proposed instability test and examined its finite sample behavior through simulation studies. Comparative performance of LongCART algorithm was evaluated empirically via simulation studies. Finally, we applied LongCART to study the longitudinal changes in choline levels among HIV-positive patients.