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We are interested in this paper in the modelling and numerical simulation of some phenomena that are observed in the context of car dynamics, in particular the appearance of persistent jams upstream critical points with no real cause of flux limitation. We shall consider the case of a stable jam on a freeway upstream an accident that took place on the opposite lane. This situation is not properly handled by most models, either micro-or macroscopic ones, since it corresponds to a phenomenon that does not have a counterpart in gas dynamics, for which only entropy solution are usually considered as physically feasible. The approach we propose consists in accounting for the very behaviour of agents in the neighbourhood of the discontinuity, and makes it possible to numerically recover in a robust way steady traffic jams.

The emergence of biobank-level datasets offers new opportunities to discover novel biomarkers and develop predictive algorithms for human disease. Here, we present an ensemble machine-learning framework (machine learning with phenotype associations, MILTON) utilizing a range of biomarkers to predict 3,213 diseases in the UK Biobank. Leveraging the UK Biobank's longitudinal health record data, MILTON predicts incident disease cases undiagnosed at time of recruitment, largely outperforming available polygenic risk scores. We further demonstrate the utility of MILTON in augmenting genetic association analyses in a phenome-wide association study of 484,230 genome-sequenced samples, along with 46,327 samples with matched plasma proteomics data. This resulted in improved signals for 88 known (P < 1 × 10 -8 ) gene-disease relationships alongside 182 gene-disease relationships that did not achieve genome-wide significance in the nonaugmented baseline cohorts. We validated these discoveries in the FinnGen biobank alongside two orthogonal machine-learning methods built for gene-disease prioritization. All extracted gene-disease associations and incident disease predictive biomarkers are publicly available ( http://milton.public.cgr.astrazeneca.com )., (© 2024. The Author(s).)

The etiology of prostate cancer, the second most common cancer in men globally, has a strong heritable component. While rare coding germline variants in several genes have been identified as risk factors from candidate gene and linkage studies, the exome-wide spectrum of causal rare variants remains to be fully explored. To more comprehensively address their contribution, we analysed data from 37,184 prostate cancer cases and 331,329 male controls from five cohorts with germline exome/genome sequencing and one cohort with imputed array data from a population enriched in low-frequency deleterious variants. Our gene-level collapsing analysis revealed that rare damaging variants in SAMHD1 as well as genes in the DNA damage response pathway ( BRCA2 , ATM and CHEK2 ) are associated with the risk of overall prostate cancer. We also found that rare damaging variants in AOX1 and BRCA2 were associated with increased severity of prostate cancer in a case-only analysis of aggressive versus non-aggressive prostate cancer. At the single-variant level, we found rare non-synonymous variants in three genes ( HOXB13 , CHEK2 , BIK ) significantly associated with increased risk of overall prostate cancer and in four genes ( ANO7 , SPDL1 , AR , TERT ) with decreased risk. Altogether, this study provides deeper insights into the genetic architecture and biological basis of prostate cancer risk and severity., Competing Interests: Competing Interests J.M., N.C., O.B., R.D., A.N., A.O., A.A., Q.W., L.A.-D., R.M., B.D., K.C., S.P., M.A.F. are current employees and/or stockholders of AstraZeneca. A.W.Z receives grant funding and consulting fees from AstraZeneca. L.A.M. is on the advisory board and holds equity interest in Convergent Therapeutics. A.M. is a former employee of AstraZeneca and current employee of GSK and a stockholder of AstraZeneca and GSK. C.H. was an employee and stockholder of AZ at the time of study. P.W.K. is a co-founder and employee of Convergent Therapeutics.

Abstract: The study of somatic mutations and the associated clonal mosaicism across the human body has transformed our understanding of aging and its links to cancer. In proliferative human tissues, stem cells compete for dominance, and those with an advantage expand clonally to outgrow their peers. In the hematopoietic system, such expansion is termed clonal hematopoiesis (CH). The forces driving competition, namely heterogeneity of the hematopoietic stem cell (HSC) pool and attrition of their environment, become increasingly prominent with age. As a result, CH becomes progressively more common through life to the point of becoming essentially ubiquitous. We are beginning to unravel the specific intracellular and extracellular factors underpinning clonal behavior, with somatic mutations in specific driver genes, inflammation, telomere maintenance, extraneous exposures, and inherited genetic variation among the important players. The inevitability of CH with age combined with its unequivocal links to myeloid cancers poses a scientific and clinical challenge. Specifically, we need to decipher the factors determining clonal behavior and develop prognostic tools to identify those at high risk of malignant progression, for whom preventive interventions may be warranted. Here, we discuss how recent advances in our understanding of the natural history of CH have provided important insights into these processes and helped define future avenues of investigation., (© 2024 American Society of Hematology. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.)

Clonal hematopoiesis (CH) is an age-related condition driven by stem and progenitor cells harboring recurrent mutations linked to myeloid neoplasms. Currently, potential effects on hematopoiesis, stem cell function and regenerative potential under stress conditions are unknown. We performed targeted DNA sequencing of 457 hematopoietic stem cell grafts collected for autologous stem cell transplantation (ASCT) in myeloma patients and correlated our findings with high-dimensional longitudinal clinical and laboratory data (26,510 data points for blood cell counts/serum values in 25 days around transplantation). We detected CHrelated mutations in 152 patients (33.3%). Since many patients (n=54) harbored multiple CH mutations in one or more genes, we applied a non-negative matrix factorization (NMF) clustering algorithm to identify genes that are commonly co-mutated in an unbiased approach. Patients with CH were assigned to one of three clusters (C1-C3) and compared to patients without CH (C0) in a gene specific manner. To study the dynamics of blood cell regeneration following ASCT, we developed a time-dependent linear mixed effect model to validate differences in blood cell count trajectories amongst different clusters. The results demonstrated that C2, composed of patients with DNMT3A and PPM1D single and co-mutated CH, correlated with reduced stem cell yields and delayed platelet count recovery following ASCT. Also, the benefit of maintenance therapy was particularly strong in C2 patients. Taken together, these data indicate an impaired regenerative potential of hematopoietic stem cell grafts harboring CH with DNMT3A and PPM1D mutations.

Gene therapy (GT) provides a potentially curative treatment option for patients with sickle cell disease (SCD); however, the occurrence of myeloid malignancies in GT clinical trials has prompted concern, with several postulated mechanisms. Here, we used whole-genome sequencing to track hematopoietic stem cells (HSCs) from six patients with SCD at pre- and post-GT time points to map the somatic mutation and clonal landscape of gene-modified and unmodified HSCs. Pre-GT, phylogenetic trees were highly polyclonal and mutation burdens per cell were elevated in some, but not all, patients. Post-GT, no clonal expansions were identified among gene-modified or unmodified cells; however, an increased frequency of potential driver mutations associated with myeloid neoplasms or clonal hematopoiesis (DNMT3A- and EZH2-mutated clones in particular) was observed in both genetically modified and unmodified cells, suggesting positive selection of mutant clones during GT. This work sheds light on HSC clonal dynamics and the mutational landscape after GT in SCD, highlighting the enhanced fitness of some HSCs harboring pre-existing driver mutations. Future studies should define the long-term fate of mutant clones, including any contribution to expansions associated with myeloid neoplasms., (© 2023. The Author(s).)

Germ line variants in the DDX41 gene have been linked to myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML) development. However, the risks associated with different variants remain unknown, as do the basis of their leukemogenic properties, impact on steady-state hematopoiesis, and links to other cancers. Here, we investigate the frequency and significance of DDX41 variants in 454 792 United Kingdom Biobank (UKB) participants and identify 452 unique nonsynonymous DNA variants in 3538 (1/129) individuals. Many were novel, and the prevalence of most varied markedly by ancestry. Among the 1059 individuals with germ line pathogenic variants (DDX41-GPV) 34 developed MDS/AML (odds ratio, 12.3 vs noncarriers). Of these, 7 of 218 had start-lost, 22 of 584 had truncating, and 5 of 257 had missense (odds ratios: 12.9, 15.1, and 7.5, respectively). Using multivariate logistic regression, we found significant associations of DDX41-GPV with MDS, AML, and family history of leukemia but not lymphoma, myeloproliferative neoplasms, or other cancers. We also report that DDX41-GPV carriers do not have an increased prevalence of clonal hematopoiesis (CH). In fact, CH was significantly more common before sporadic vs DDX41-mutant MDS/AML, revealing distinct evolutionary paths. Furthermore, somatic mutation rates did not differ between sporadic and DDX41-mutant AML genomes, ruling out genomic instability as a driver of the latter. Finally, we found that higher mean red cell volume (MCV) and somatic DDX41 mutations in blood DNA identify DDX41-GPV carriers at increased MDS/AML risk. Collectively, our findings give new insights into the prevalence and cognate risks associated with DDX41 variants, as well as the clonal evolution and early detection of DDX41-mutant MDS/AML., (© 2023 by The American Society of Hematology.)

Integrating human genomics and proteomics can help elucidate disease mechanisms, identify clinical biomarkers and discover drug targets 1-4 . Because previous proteogenomic studies have focused on common variation via genome-wide association studies, the contribution of rare variants to the plasma proteome remains largely unknown. Here we identify associations between rare protein-coding variants and 2,923 plasma protein abundances measured in 49,736 UK Biobank individuals. Our variant-level exome-wide association study identified 5,433 rare genotype-protein associations, of which 81% were undetected in a previous genome-wide association study of the same cohort 5 . We then looked at aggregate signals using gene-level collapsing analysis, which revealed 1,962 gene-protein associations. Of the 691 gene-level signals from protein-truncating variants, 99.4% were associated with decreased protein levels. STAB1 and STAB2, encoding scavenger receptors involved in plasma protein clearance, emerged as pleiotropic loci, with 77 and 41 protein associations, respectively. We demonstrate the utility of our publicly accessible resource through several applications. These include detailing an allelic series in NLRC4, identifying potential biomarkers for a fatty liver disease-associated variant in HSD17B13 and bolstering phenome-wide association studies by integrating protein quantitative trait loci with protein-truncating variants in collapsing analyses. Finally, we uncover distinct proteomic consequences of clonal haematopoiesis (CH), including an association between TET2-CH and increased FLT3 levels. Our results highlight a considerable role for rare variation in plasma protein abundance and the value of proteogenomics in therapeutic discovery., (© 2023. The Author(s).)

The myeloid neoplasms encompass acute myeloid leukemia, myelodysplastic syndromes and myeloproliferative neoplasms. Most cases arise from the shared ancestor of clonal hematopoiesis (CH). Here we analyze data from 454,340 UK Biobank participants, of whom 1,808 developed a myeloid neoplasm 0-15 years after recruitment. We describe the differences in CH mutational landscapes and hematology/biochemistry test parameters among individuals that later develop myeloid neoplasms (pre-MN) versus controls, finding that disease-specific changes are detectable years before diagnosis. By analyzing differences between 'pre-MN' and controls, we develop and validate Cox regression models quantifying the risk of progression to each myeloid neoplasm subtype. We construct 'MN-predict', a web application that generates time-dependent predictions with the input of basic blood tests and genetic data. Our study demonstrates that many individuals that develop myeloid neoplasms can be identified years in advance and provides a framework for disease-specific prognostication that will be of substantial use to researchers and physicians., (© 2023. The Author(s).)

Individuals with age-related clonal hematopoiesis (CH) are at greater risk for hematologic malignancies and cardiovascular diseases. However, predictive preclinical animal models to recapitulate the spectrum of human CH are lacking. Through error-corrected sequencing of 56 human CH/myeloid malignancy genes, we identified natural CH driver mutations in aged rhesus macaques matching genes somatically mutated in human CH, with DNMT3A mutations being the most frequent. A CH model in young adult macaques was generated via autologous transplantation of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated gene-edited hematopoietic stem and progenitor cells (HSPCs), targeting the top human CH genes with loss-of-function (LOF) mutations. Long-term follow-up revealed reproducible and significant expansion of multiple HSPC clones with heterozygous TET2 LOF mutations, compared with minimal expansion of clones bearing other mutations. Although the blood counts of these CH macaques were normal, their bone marrows were hypercellular and myeloid-predominant. TET2-disrupted myeloid colony-forming units isolated from these animals showed a distinct hyperinflammatory gene expression profile compared with wild type. In addition, mature macrophages purified from the CH macaques showed elevated NLRP3 inflammasome activity and increased interleukin-1β (IL-1β) and IL-6 production. The model was used to test the impact of IL-6 blockage by tocilizumab, documenting a slowing of TET2-mutated expansion, suggesting that interruption of the IL-6 axis may remove the selective advantage of mutant HSPCs. These findings provide a model for examining the pathophysiology of CH and give insights into potential therapeutic interventions.

Clonal hematopoiesis (CH), the clonal expansion of a blood stem cell and its progeny driven by somatic driver mutations, affects over a third of people, yet remains poorly understood. Here we analyze genetic data from 200,453 UK Biobank participants to map the landscape of inherited predisposition to CH, increasing the number of germline associations with CH in European-ancestry populations from 4 to 14. Genes at new loci implicate DNA damage repair (PARP1, ATM, CHEK2), hematopoietic stem cell migration/homing (CD164) and myeloid oncogenesis (SETBP1). Several associations were CH-subtype-specific including variants at TCL1A and CD164 that had opposite associations with DNMT3A- versus TET2-mutant CH, the two most common CH subtypes, proposing key roles for these two loci in CH development. Mendelian randomization analyses showed that smoking and longer leukocyte telomere length are causal risk factors for CH and that genetic predisposition to CH increases risks of myeloproliferative neoplasia, nonhematological malignancies, atrial fibrillation and blood epigenetic ageing., (© 2022. The Author(s).)

Clonal expansions driven by somatic mutations become pervasive across human tissues with age, including in the haematopoietic system, where the phenomenon is termed clonal haematopoiesis 1-4 . The understanding of how and when clonal haematopoiesis develops, the factors that govern its behaviour, how it interacts with ageing and how these variables relate to malignant progression remains limited 5,6 . Here we track 697 clonal haematopoiesis clones from 385 individuals 55 years of age or older over a median of 13 years. We find that 92.4% of clones expanded at a stable exponential rate over the study period, with different mutations driving substantially different growth rates, ranging from 5% (DNMT3A and TP53) to more than 50% per year (SRSF2 P95H ). Growth rates of clones with the same mutation differed by approximately ±5% per year, proportionately affecting slow drivers more substantially. By combining our time-series data with phylogenetic analysis of 1,731 whole-genome sequences of haematopoietic colonies from 7 individuals from an older age group, we reveal distinct patterns of lifelong clonal behaviour. DNMT3A-mutant clones preferentially expanded early in life and displayed slower growth in old age, in the context of an increasingly competitive oligoclonal landscape. By contrast, splicing gene mutations drove expansion only later in life, whereas TET2-mutant clones emerged across all ages. Finally, we show that mutations driving faster clonal growth carry a higher risk of malignant progression. Our findings characterize the lifelong natural history of clonal haematopoiesis and give fundamental insights into the interactions between somatic mutation, ageing and clonal selection., (© 2022. The Author(s).)

Age-related change in human haematopoiesis causes reduced regenerative capacity 1 , cytopenias 2 , immune dysfunction 3 and increased risk of blood cancer 4-6 , but the reason for such abrupt functional decline after 70 years of age remains unclear. Here we sequenced 3,579 genomes from single cell-derived colonies of haematopoietic cells across 10 human subjects from 0 to 81 years of age. Haematopoietic stem cells or multipotent progenitors (HSC/MPPs) accumulated a mean of 17 mutations per year after birth and lost 30 base pairs per year of telomere length. Haematopoiesis in adults less than 65 years of age was massively polyclonal, with high clonal diversity and a stable population of 20,000-200,000 HSC/MPPs contributing evenly to blood production. By contrast, haematopoiesis in individuals aged over 75 showed profoundly decreased clonal diversity. In each of the older subjects, 30-60% of haematopoiesis was accounted for by 12-18 independent clones, each contributing 1-34% of blood production. Most clones had begun their expansion before the subject was 40 years old, but only 22% had known driver mutations. Genome-wide selection analysis estimated that between 1 in 34 and 1 in 12 non-synonymous mutations were drivers, accruing at constant rates throughout life, affecting more genes than identified in blood cancers. Loss of the Y chromosome conferred selective benefits in males. Simulations of haematopoiesis, with constant stem cell population size and constant acquisition of driver mutations conferring moderate fitness benefits, entirely explained the abrupt change in clonal structure in the elderly. Rapidly decreasing clonal diversity is a universal feature of haematopoiesis in aged humans, underpinned by pervasive positive selection acting on many more genes than currently identified., (© 2022. The Author(s).)

Although acquisition of leukemia-associated somatic mutations by 1 or more hematopoietic stem cells is inevitable with advancing age, its consequences are highly variable, ranging from clinically silent clonal hematopoiesis (CH) to leukemic progression. To investigate the influence of heritable factors on CH, we performed deep targeted sequencing of blood DNA from 52 monozygotic (MZ) and 27 dizygotic (DZ) twin pairs (aged 70-99 years). Using this highly sensitive approach, we identified CH (variant allele frequency ≥0.5%) in 62% of individuals. We did not observe higher concordance for CH within MZ twin pairs as compared with that within DZ twin pairs, or to that expected by chance. However, we did identify 2 MZ pairs in which both twins harbored identical rare somatic mutations, suggesting a shared cell of origin. Finally, in 3 MZ twin pairs harboring mutations in the same driver genes, serial blood samples taken 4 to 5 years apart showed substantial twin-to-twin variability in clonal trajectories. Our findings propose that the inherited genome does not exert a dominant influence on the behavior of adult CH and provide evidence that CH mutations may be acquired in utero., (© 2020 by The American Society of Hematology.)

Background: Some therapeutic agents in oncology can be causally associated with specific cardiovascular events including QT/QTc interval prolongation. We investigated the effect of multiple dosing of the oral poly (ADP-ribose)-polymerase (PARP) inhibitor, olaparib (tablet formulation) on QT/QTc interval., Methods: Two phase I, open-label, three-part studies (NCT01921140 [study 4] and NCT01900028 [study 7]) were conducted in adults with refractory/resistant advanced solid tumours. In both studies, parts A and B assessed the QT/QTc interval effects of single-dose oral olaparib 100 (study 4) or 300 (study 7) mg and multiple-dose olaparib 300 mg bid for 5 days, respectively, while part C evaluated continued access to olaparib for additional safety analyses. An ANCOVA model tested the primary objective of multiple-dose effects of olaparib on QT interval corrected using Fridericia's formula (QTcF)., Results: Data from 119 and 109 patients were pooled from parts A and B, respectively, for QT/QTc analysis. At pre-dose and up to 12 h post-dose, the upper limits of the 90 % confidence intervals (CIs) for the difference in QTcF least squares means after olaparib multiple dosing versus control (day -1) were <10 ms, suggesting a lack of clinically relevant effect on cardiac repolarization. A slight shortening of QTcF was observed at most time points versus control. QTcF results for the individual studies and single-dose olaparib paralleled the primary multiple-dose pooled analysis, with upper limits of the 90 % CIs < 10 ms., Conclusion: Olaparib tablets administered as multiple or single doses had no clinically significant effect on QT/QTc interval.

First-in-human (FIH) studies with AZD3514, a selective androgen receptor (AR) down-regulator, showed decreases of >30% in the prostate-specific antigen (PSA) in some patients. A modeling approach was adopted to understand these observations and define the optimum clinical use hypothesis for AZD3514 for clinical testing. Initial empirical modeling showed that only baseline PSA correlated significantly with this biological response, whereas drug concentration did not. To identify the mechanistic cause of this observation, a mechanism-based model was first developed, which described the effects of AZD3514 on AR protein and PSA mRNA levels in LNCaP cells with and without dihydrotestosterone (DHT). Second, the mechanism-based model was linked to a population pharmacokinetic (PK) model; PSA effects of clinical doses were subsequently simulated under different clinical conditions. This model was used to adjust the design of the ongoing clinical FIH study and direct the backup program., (© 2016 The Authors CPT: Pharmacometrics & Systems Pharmacology published by Wiley Periodicals, Inc. on behalf of American Society for Clinical Pharmacology and Therapeutics.)

Background and Objectives: Oral risperidone is licensed globally for the treatment of several psychiatric disorders in children, adolescents and adults. The pharmacokinetic profile of risperidone is well documented in adults. In this study, the pharmacokinetics of oral risperidone in children and adolescents were investigated along with population pharmacokinetics in paediatric and adult subjects., Methods: The pharmacokinetics of oral risperidone in children and adolescents were investigated through non-compartmental analysis (paediatric phase I study; n = 24) and population pharmacokinetic analysis using nonlinear mixed-effects modelling software (NONMEM) on a pooled database including both paediatric (n = 304) and adult (n = 476) data. Monte Carlo simulations were performed to evaluate the relevance of the effects of covariates on the plasma exposure of the active antipsychotic fraction., Results: Non-compartmental pharmacokinetic analysis showed that, after correcting doses for bodyweight, plasma exposure was comparable between children and adolescents and in line with historical adult data. Pooled population pharmacokinetic analysis, using a priori allometric scaling of the clearance and volume of distribution, showed that apparent renal clearance of the active antipsychotic fraction was 0.96 L/h and apparent metabolic clearance was 4.26 L/h for a typical patient weighing 62 kg, aged 18.1 years, with a median creatinine clearance of 117.6 mL/min. For a typical child (11 years, 39 kg), adolescent (15 years, 60 kg) and adult (33 years, 70 kg), the apparent total oral clearance values were 4.35, 5.30 and 5.04 L/h, respectively. None of the tested demographic or biochemical characteristics were found to have a relevant effect on any of the pharmacokinetic parameters of risperidone and the active antipsychotic fraction., Conclusion: Population pharmacokinetics and Monte Carlo simulations demonstrated similar pharmacokinetics of risperidone in children, adolescents and adults.

WST11, a novel generation of photo sensitizers to be used for vascular-targeted phototherapy (VTP), is effective at short interval between injection and illumination and it is expected to enable selective destruction of neovasculature with minimal side effects or skin photo toxicity. This study was conducted to evaluate the clinical and laboratory safety, tolerability and pharmacokinetic profile of WST11 given as a single intravenous administration (1.25, 2.5, 5, 7.5, 10, or 15 mg/kg) during an escalating dose study in healthy male subjects. This article describes WST11 population pharmacokinetic modeling and simulations performed to optimize the IV infusion-dosing regimen in combination with illumination, the target PK profile being plateau concentrations during approximately 30 min. The study included 42 healthy male subjects, administered 1.25, 2.5, 5, 7.5, 10, or 15 mg/kg as a 10-min IV infusion. A population pharmacokinetic model was developed using nonlinear mixed effects modeling (NONMEM). Monte Carlo simulations of the population PK dataset (NONMEM) were performed to select series of dosing regimen which would result in a plateau of concentration lasting at least 30 min and allow laser illumination. A two-compartment model with nonlinear elimination best described the data. No demographic factor was shown to affect the WST11 pharmacokinetics. The clearance was shown to decreases with the dose administered, ranging from 6 L/h (dose of 79 mg) to 2 L/h (dose of 1110 mg). The duration of the infusion was estimated at 12 min. The volume of distribution of the central compartment was 3 L and the volume of the peripheral compartment was 1.15 L. The apparent inter-compartmental clearance was 0.137 L/h. The between subjects variability on clearance and on volume was low. Residual variability was moderate with a CV of 21%. Due to the dose effect on clearance and the rapid elimination, simulations showed that different dosing inputs are necessary: for 5 and 10 mg/kg BW, a sufficiently good dosing scenario is to administer 80% of the dose over 5 min, 15% over 10 min and the remaining 5% over 10 min. For lower doses, the sequence 70% in 5 min/20% in 10 min/10% in 10 min is preferable. The pharmacokinetic profile of WST11 by IV administration would allow a treatment by laser illumination in good clinical conditions using controlled infusions. The study results do not indicate that the dose should be adjusted for body size. The only factor that determines the drug input profile is the dose level, since the elimination half-life decreases when the dose administered increases. The use of the population PK model for simulations has shown that, at dose level of 5 mg/kg or more, a loading dose of 80% dose given over 5 min followed by 15% of dose during 10 min and remained dose to give over 10 min would result in a favorable PK profile., ((c) 2007 Wiley-Liss, Inc.)

Azathioprine metabolism is influenced by activity of the enzyme thiopurine S-methyltransferase (TPMT), which varies markedly between individuals. In this study we examined the influence of TPMT gene polymorphisms on azathioprine dose 1 year after renal transplantation. TPMT coding and promoter genotypes were determined using PCR-based assays. Azathioprine dose, white cell count, and intercurrent events throughout the first year after renal transplantation were ascertained from contemporaneous clinical notes. All patients analysed ( n=172) received an initial azathioprine dose of 1.5 mg/kg per day. Twelve individuals with one variant TPMT coding allele were detected (*3A n=11, *3C n=1). Of these, 58% required azathioprine dose reduction because of leucopenia, compared to only 30% of homozygous wild-type patients ( P=0.04). A significant correlation between the presence of >/=11 variable number tandem repeats (VNTRs) in the TPMT promoter and reduction in azathioprine dose was also identified ( P=0.001). We concluded that when azathioprine is administered at an initial dose of 1.5 mg/kg per day, both coding and promoter TPMT polymorphisms influence the dose tolerated.