Pharmacokinetics of Briquilimab as a Conditioning Agent for Hematopoietic Stem Cell Transplantation in Patients with Severe Combined Immunodeficiency, Myelodysplastic Syndrome, or Acute Myeloid Leukemia.

BACKGROUND: For successful engraftment of donor hematopoietic stem cells (HSC), conditioning with chemotherapy and/or radiation prior to hematopoietic cell transplantation (HCT) has been required to open marrow niche space and minimize the risk of immune rejection. Briquilimab, a humanized IgG1 monoclonal antibody that blocks the interaction between the c-Kit receptor and stem cell factor on various C-Kit expressing tissues including HSC, is a potential nonmyeloablative conditioning agent in clinical development for patients with severe combined immunodeficiency (SCID), myelodysplastic syndromes (MDS), and acute myeloid leukemia (AML). OBJECTIVE(S): This study aimed to characterize pharmacokinetics (PK) and develop a population PK model of briquilimab after single intravenous infusions of 4 different doses in patients with SCID, MDS, or AML receiving HCT. STUDY DESIGN: The PK data was collected from 2 different studies: JAS-BMT-CP-001 and JSP-CP-003. JAS-BMT-CP-001 is a phase 1/2 open-label study of briquilimab as a conditioning agent prior to allogenic HCT in SCID patients. The participants received single intravenous infusions of 0.1, 0.3, 0.6, or 1.0 mg/kg. JSP-CP-003 was a phase 1a/b open-label study of briquilimab in combination with a standard conditioning regimen of low dose total body irradiation and fludarabine in MDS or AML subjects undergoing HCT. The participants received a single intravenous dose of 0.6 mg/kg briquilimab. In both studies, briquilimab PK samples were obtained at pre-treatment, 5 minutes post-end of infusion, 4- and 24-hours post-start of infusion, any time between 2 days and 30 days post-infusion, and on the day of HCT prior to donor cell infusion.The population PK model was developed using the PK data from these 2 clinical studies, and the effect of participants' baseline characteristics on the briquilimab PK was evaluated. PK simulations were performed using the developed PK model to calculate the time to reach target concentrations for HCT. RESULTS: A total of 49 participants (21 SCID adult and pediatric participants with a median age of 12 years and 28 MDS/AML adult participants with a median age of 70 years) were included in the PK analysis. A two-compartment model with combined linear and non-linear elimination best described the PK of briquilimab. Body weight was determined as the sole covariate of the PK parameters among the explored covariates. For a typical subject with a body weight 70 kg, the estimated parameters for clearance, maximum metabolic rate of Michaelis Menten elimination, Michaelis Menten constant, central volume, peripheral volume, and intercompartmental clearance were 17.6 mL/hr, 51434.8 ng/hr, 71.5 ng/mL, 3444.0 mL, 1613.3 mL, and 21.2 mL/hr, respectively. The median time to reach target concentrations of 500, 1000, and 2000 ng/mL after a single dose of 0.6 mg/kg was calculated as 12.3, 10.4, and 7.7 days, respectively. CONCLUSIONS: The PK of intravenous briquilimab was characterized in subjects with SCID, MDS, or AML receiving HCT, and a population PK model was developed to estimate briquilimab clearance to use as a guide to the timing of donor cell infusion post-briquilimab. Body weight was identified as a significant covariate on elimination and volume of distribution of briquilimab.

Cyclophosphamide and thiotepa increases risk of transplant-associated thrombotic microangiopathy.

BACKGROUND: Transplant associated thrombotic microangiopathy (TA-TMA) is a complication of hematopoietic cell transplant (HCT) associated with endothelial injury resulting in severe end organ damage, acute and long-term morbidity, and mortality. Myeloablative conditioning is a known risk factor, though specific causative agents have not been identified. We hypothesized that the combination of cyclophosphamide and thiotepa (CY+TT) is particularly toxic to the endothelium, placing patients at elevated risk for TA-TMA. METHODS: We conducted a retrospective review of pediatric and young adult patients who received conditioned autologous and allogeneic HCT between 2012 and August 2023 at UCSF Benioff Children's Hospital, San Francisco. We excluded patients undergoing gene therapy or triple tandem transplants for brain tumors. Neuroblastoma tandem transplants were classified a single transplant occurrence. High dose N-acetylcysteine (NAC) prophylaxis was incorporated into the institutional standard of care from December 2016-May 2019 and May 2022-August 2023. Defibrotide was given prophylactically to patients deemed high-risk for sinusoidal obstruction syndrome (SOS) per institutional guidelines or on clinical trial NCT#02851407 for SOS prophylaxis or NCT#03384693 for TA-TMA prophylaxis. Kaplan-Meier analysis was used to estimate the 1-year cumulative incidence of TA-TMA. Univariate analysis was performed for each of the potential risk factors of interest using log-rank tests and bivariate analysis with Cox regression models using backward selection and hazard ratios were built using all covariates with a univariate p-value <0.2 for allogeneic HCT. SPSS (v29) was used to estimate all summary statistics, cumulative incidences, and uni- and bi-variate analyses. RESULTS: A total of 558 transplants were performed with 43 patients developing TA-TMA, for a 1-year cumulative incidence of 8.6% (95% CI, 5.9-11.3%) and 7.2% (95% CI, 2.9-11.5%) in allogeneic and autologous HCTs, respectively (p=0.62). In allogeneic recipients (n=417), the 1-year cumulative incidence of TA-TMA with CY+TT as part of conditioning was 35.7% (95% CI, 15.7-55.7%) compared to 11.7% (95% CI, 7.2-16.2%) with either CY or TT alone, and 1.2% (95% CI, 0-2.8%) if neither agent was included in the conditioning regimen (p<0.001). Use of either CY or TT (HR=10.14; p=0.002) or CY+TT (HR=35.93; p<0.001), viral infections (HR=4.3; p=0.017) and fungal infections (HR=2.98; p=0.027) were significant factors resulting in increased risk for developing TA-TMA. In subjects undergoing autologous HCT (n=141), the 1-year cumulative incidence of TA-TMA with CY+TT was 19.6% (95% CI, 8.8-30.6%) while TA-TMA did not occur in patients receiving either CY or TT alone or when neither were included (p<0.001). TA-TMA occurred only in patients with neuroblastoma receiving CY+TT as part of their conditioning. For autologous patients who received CY+TT, those who were CMV seronegative at the time of HCT had an incidence of TA-TMA of 6.7% (95% CI, 0.1-15.7%) compared to 38.1% (95% CI, 35-41.2%) for those CMV seropositive (p=0.007). CONCLUSIONS: These data show that CY or TT alone or in combination as part of pre-transplant conditioning prior to HCT increase the incidence of TA-TMA. Alternative conditioning excluding the combination of CY+TT should be considered whenever possible to limit the development of TA-TMA.

Diagnosing and Grading of Sinusoidal Obstructive Syndrome after Hematopoietic Stem Cell Transplant of Children, Adolescent and Young Adults treated in a Pediatric Institution with Pediatric Protocols.

Sinusoidal obstructive syndrome (SOS), or veno-occlusive disease, of the liver has been recognized as a complex, life-threatening complication in the posthematopoietic stem cell transplant (HSCT) setting. The diagnostic criteria for SOS have evolved over the last several decades with a greater understanding of the underlying pathophysiology, with 2 recent diagnostic criteria introduced in 2018 (European Society of Bone Marrow Transplant [EBMT] criteria) and 2020 (Cairo criteria). We sought out to evaluate the performance characteristics in diagnosing and grading SOS in pediatric patients of the 4 different diagnostic criteria (Baltimore, Modified Seattle, EBMT, and Cairo) and severity grading systems (defined by the EBMT and Cairo criteria). Retrospective chart review of children, adolescent, and young adults who underwent conditioned autologous and allogeneic HSCT between 2017 and 2021 at a single pediatric institution. A total of 250 consecutive patients underwent at least 1 HSCT at UCSF Benioff Children's Hospital San Francisco for a total of 307 HSCT. The day 100 cumulative incidence of SOS was 12.1%, 21.1%, 28.4%, and 28.4% per the Baltimore, Modified Seattle, EBMT, and Cairo criteria, respectively (P < .001). We found that patients diagnosed with grade ≥4 SOS per the Cairo criteria were more likely to be admitted to the Pediatric Intensive Care Unit (92% versus 58%, P = .035) and intubated (85% versus 32%, P = .002) than those diagnosed with grade ≥4 per EBMT criteria. Age <3 years-old (HR 1.76, 95% [1.04 to 2.98], P = .036), an abnormal body mass index (HR 1.69, 95% [1.06 to 2.68], P = .027), and high-risk patients per our institutional guidelines (HR 1.68, 95% [1.02 to 2.76], P = .041) were significantly associated with SOS per the Cairo criteria. We demonstrate that age <3 years, abnormal body mass index, and other high-risk criteria associate strongly with subsequent SOS development. Patients with moderate to severe SOS based on Cairo severity grading system may correlate better with clinical course based on ICU admissions and intubations when compared to the EBMT severity grading system.

Maximum a posteriori Bayesian methods out-perform non-compartmental analysis for busulfan precision dosing.

Dose personalization improves patient outcomes for many drugs with a narrow therapeutic index and high inter-individuality variability, including busulfan. Non-compartmental analysis (NCA) and model-based methods like maximum a posteriori Bayesian (MAP) approaches are two methods routinely used for dose optimization. These approaches vary in how they estimate patient-specific pharmacokinetic parameters to inform a dose and the impact of these differences is not well-understood. Using busulfan as an example application and area under the concentration-time curve (AUC) as a target exposure metric, these estimation methods were compared using retrospective patient data (N = 246) and simulated precision dosing treatment courses. NCA was performed with or without peak extension, and MAP Bayesian estimation was performed using either the one-compartment Shukla model or the two-compartment McCune model. All methods showed good agreement on real-world data (correlation coefficients of 0.945-0.998) as assessed by Bland-Altman plots, although agreement between NCA and MAP methods was higher during the first dosing interval (0.982-0.994) compared to subsequent dosing intervals (0.918-0.938). In dose adjustment simulations, both NCA and MAP estimated high target attainment (> 98%) although true simulated target attainment was lower for NCA (63-66%) versus MAP (91-93%). The largest differences in AUC estimation were due to different assumptions for the shape of the concentration curve during the infusion phase, followed by how the methods considered time-dependent clearance and concentration-time points collected in earlier intervals. In conclusion, although AUC estimates between the two methods showed good correlation, in a simulated study, MAP lead to higher target attainment. When changing from one method to another, or changing infusion duration and other factors, optimum estimated exposure targets may require adjusting to maintain a consistent exposure.

Effect of Clemastine on Neurophysiological Outcomes in an Ovine Model of Neonatal Hypoxic-Ischemic Encephalopathy.

Originally approved by the U.S. Food and Drug Administration (FDA) for its antihistamine properties, clemastine can also promote white matter integrity and has shown promise in the treatment of demyelinating diseases such as multiple sclerosis. Here, we conducted an in-depth analysis of the feasibility, safety, and neuroprotective efficacy of clemastine administration in near-term lambs (n = 25, 141-143 days) following a global ischemic insult induced via an umbilical cord occlusion (UCO) model. Lambs were randomly assigned to receive clemastine or placebo postnatally, and outcomes were assessed over a six-day period. Clemastine administration was well tolerated. While treated lambs demonstrated improvements in inflammatory scores, their neurodevelopmental outcomes were unchanged.

Perinatal Azithromycin Provides Limited Neuroprotection in an Ovine Model of Neonatal Hypoxic-Ischemic Encephalopathy.

BACKGROUND: Hypoxic-ischemic brain injury/encephalopathy affects about 1.15 million neonates per year, 96% of whom are born in low- and middle-income countries. Therapeutic hypothermia is not effective in this setting, possibly because injury occurs significantly before birth. Here, we studied the pharmacokinetics, safety, and efficacy of perinatal azithromycin administration in near-term lambs following global ischemic injury to support earlier treatment approaches. METHODS: Ewes and their lambs of both sexes (n=34, 141-143 days) were randomly assigned to receive azithromycin or placebo before delivery as well as postnatally. Lambs were subjected to severe global hypoxia-ischemia utilizing an acute umbilical cord occlusion model. Outcomes were assessed over a 6-day period. RESULTS: While maternal azithromycin exhibited relatively low placental transfer, azithromycin-treated lambs recovered spontaneous circulation faster following the initiation of cardiopulmonary resuscitation and were extubated sooner. Additionally, peri- and postnatal azithromycin administration was well tolerated, demonstrating a 77-hour plasma elimination half-life, as well as significant accumulation in the brain and other tissues. Azithromycin administration resulted in a systemic immunomodulatory effect, demonstrated by reductions in proinflammatory IL-6 (interleukin-6) levels. Treated lambs exhibited a trend toward improved neurodevelopmental outcomes while histological analysis revealed that azithromycin supported white matter preservation and attenuated inflammation in the cingulate and parasagittal cortex. CONCLUSIONS: Perinatal azithromycin administration enhances neonatal resuscitation, attenuates neuroinflammation, and supports limited improvement of select histological outcomes in an ovine model of hypoxic-ischemic brain injury/encephalopathy.

Lentiviral Gene Therapy for Artemis-Deficient SCID.

BACKGROUND: The DNA-repair enzyme Artemis is essential for rearrangement of T- and B-cell receptors. Mutations in DCLRE1C, which encodes Artemis, cause Artemis-deficient severe combined immunodeficiency (ART-SCID), which is poorly responsive to allogeneic hematopoietic-cell transplantation. METHODS: We carried out a phase 1-2 clinical study of the transfusion of autologous CD34+ cells, transfected with a lentiviral vector containing DCLRE1C, in 10 infants with newly diagnosed ART-SCID. We followed them for a median of 31.2 months. RESULTS: Marrow harvest, busulfan conditioning, and lentiviral-transduced CD34+ cell infusion produced the expected grade 3 or 4 adverse events. All the procedures met prespecified criteria for feasibility at 42 days after infusion. Gene-marked T cells were detected at 6 to 16 weeks after infusion in all the patients. Five of 6 patients who were followed for at least 24 months had T-cell immune reconstitution at a median of 12 months. The diversity of T-cell receptor ß chains normalized by 6 to 12 months. Four patients who were followed for at least 24 months had sufficient B-cell numbers, IgM concentration, or IgM isohemagglutinin titers to permit discontinuation of IgG infusions. Three of these 4 patients had normal immunization responses, and the fourth has started immunizations. Vector insertion sites showed no evidence of clonal expansion. One patient who presented with cytomegalovirus infection received a second infusion of gene-corrected cells to achieve T-cell immunity sufficient for viral clearance. Autoimmune hemolytic anemia developed in 4 patients 4 to 11 months after infusion; this condition resolved after reconstitution of T-cell immunity. All 10 patients were healthy at the time of this report. CONCLUSIONS: Infusion of lentiviral gene-corrected autologous CD34+ cells, preceded by pharmacologically targeted low-exposure busulfan, in infants with newly diagnosed ART-SCID resulted in genetically corrected and functional T and B cells. (Funded by the California Institute for Regenerative Medicine and the National Institute of Allergy and Infectious Diseases; ClinicalTrials.gov number, NCT03538899.).

Prospective Validation and Refinement of a Population Pharmacokinetic Model of Fludarabine in Children and Young Adults Undergoing Hematopoietic Cell Transplantation.

Fludarabine is a nucleoside analog with antileukemic and immunosuppressive activity commonly used in allogeneic hematopoietic cell transplantation (HCT). Several fludarabine population pharmacokinetic (popPK) and pharmacodynamic models have been published enabling the movement towards precision dosing of fludarabine in pediatric HCT; however, developed models have not been validated in a prospective cohort of patients. In this multicenter pharmacokinetic study, fludarabine plasma concentrations were collected via a sparse-sampling strategy. A fludarabine popPK model was evaluated and refined using standard nonlinear mixed effects modelling techniques. The previously described fludarabine popPK model well-predicted the prospective fludarabine plasma concentrations. Individuals who received model-based dosing (MBD) of fludarabine achieved significantly more precise overall exposure of fludarabine. The fludarabine popPK model was further improved by both the inclusion of fat-free mass instead of total body weight and a maturation function on fludarabine clearance. The refined popPK model is expected to improve dosing recommendations for children younger than 2 years and patients with higher body mass index. Given the consistency of fludarabine clearance and exposure across its multiple days of administration, therapeutic drug monitoring is not likely to improve targeted exposure attainment.

Quantification of N, N' N"-triethylenethiophosphoramide, N, N"-triethylenephosphoramide, cyclophosphamide, and 4-hydroxy-cyclophosphamide in microvolume human plasma to support neonatal and pediatric drug studies.

N, N' N"-triethylenethiophosphoramide (thiotepa) and cyclophosphamide (CP) are alkylating agents used for a variety of malignant and non-malignant disorders. Both drugs are metabolized by cytochrome P450 enzymes to form active metabolites. To support pharmacokinetic studies of thiotepa and CP in children, we sought to develop assays to determine parent drug and metabolite concentration in small volume plasma samples. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used for assay development. CP metabolite 4-hydroxycyclophosphamide (4OHCP) was converted to the more stable semicarbazone derivative (4OHCP-SCZ) for quantitation. Samples (10 µL) were extracted by solid-phase extraction and injected onto the LC-MS/MS system equipped with a pentafluorophenyl reverse phase column (2.1 × 50 mm, 2.7 µm). Electrospray ionization in positive mode was used for detection. Multiple reaction monitoring of the precursor-to-product ion transitions m/z 190→147 for thiotepa, 174→131 for tepa, 261→233 for CP, and 334→221 for 4OHCP-SCZ was selected for quantification. The ion transitions m/z 202→155 for thiotepa-d12, 186→139 for tepa-d12, 267→237 for CP-d4, and 340→114 for 4OHCP-d4-SCZ were selected for the internal standard (IS) corresponding to each analyte. The less abundant IS ions from 37Cl were used for CP-d4 and 4OHCP-d4-SCZ to overcome the cross-talk interference from the analytes. Under optimized conditions, retention times were 0.67 min for tepa and its IS, 2.50 min for thiotepa and its IS, 2.52 min for 4OHCP-SCZ and its IS, and 2.86 min for CP and its IS. Total run time was 5 min per sample. The calibration ranges were 2.5-2,000ng/mL for thiotepa and tepa, 20-10,000ng/mL for CP and 20-5,000 ng/mL for 4OHCP; Dilution integrity for samples above the calibration range was validated with 10-fold dilution for thiotepa/tepa and 20-fold dilution for CP/4OHCP. Recoveries ranged from 86.3-93.4% for thiotepa, 86.3-89.0% for tepa, 90.2-107% for CP, and 99.3-115% for 4OHCP-SCZ. The IS normalized matrix effect was within (100±7) % for all 4 analytes. Plasma samples at room temperature were stable for at least 60 hours for thiotepa, 6 days for tepa, and 24 hours for CP and 4OHCP-SCZ. Plasma samples for thiotepa/tepa were stable after 4 freeze-thaw cycles, and for CP/4OHCP-SCZ were stable after 3 freeze-thaw cycles. The assays were validated and applied to clinical studies requiring small sample volumes.

Defining Longer-Term Outcomes in an Ovine Model of Moderate Perinatal Hypoxia-Ischemia.

Hypoxic-ischemic encephalopathy (HIE) is the leading cause of neonatal morbidity and mortality worldwide. Approximately 1 million infants born with HIE each year survive with cerebral palsy and/or serious cognitive disabilities. While infants born with mild and severe HIE frequently result in predictable outcomes, infants born with moderate HIE exhibit variable outcomes that are highly unpredictable. Here, we describe an umbilical cord occlusion (UCO) model of moderate HIE with a 6-day follow-up. Near-term lambs (n = 27) were resuscitated after the induction of 5 min of asystole. Following recovery, lambs were assessed to define neurodevelopmental outcomes. At the end of this period, lambs were euthanized, and brains were harvested for histological analysis. Compared with prior models that typically follow lambs for 3 days, the observation of neurobehavioral outcomes for 6 days enabled identification of animals that recover significant neurological function. Approximately 35% of lambs exhibited severe motor deficits throughout the entirety of the 6-day course and, in the most severely affected lambs, developed spastic diparesis similar to that observed in infants who survive severe neonatal HIE (severe, UCOs). Importantly, and similar to outcomes in human neonates, while initially developing significant acidosis and encephalopathy, the remainder of the lambs in this model recovered normal motor activity and exhibited normal neurodevelopmental outcomes by 6 days of life (improved, UCOi). The UCOs group exhibited gliosis and inflammation in both white and gray matters, oligodendrocyte loss, neuronal loss, and cellular death in the hippocampus and cingulate cortex. While the UCOi group exhibited more cellular death and gliosis in the parasagittal cortex, they demonstrated more preserved white matter markers, along with reduced markers of inflammation and lower cellular death and neuronal loss in Ca3 of the hippocampus compared with UCOs lambs. Our large animal model of moderate HIE with prolonged follow-up will help further define pathophysiologic drivers of brain injury while enabling identification of predictive biomarkers that correlate with disease outcomes and ultimately help support development of therapeutic approaches to this challenging clinical scenario.

Population Pharmacokinetics of Melphalan for Pediatric Patients Undergoing Hematopoietic Cell Transplantation.

Melphalan is an alkylating agent used as part of conditioning prior to pediatric hematopoietic cell transplantation (HCT). We performed a single-center, prospective pharmacokinetic study of 37 pediatric patients undergoing HCT from March 2015 to 2019. The primary objective was to develop and validate a population pharmacokinetic model for melphalan in a diverse group of pediatric HCT recipients. Nonlinear mixed-effects modeling was implemented to describe plasma concentration-time data of melphalan. A 2-compartment, proportional error model with weight on clearance best fit the data. Final parameter estimates were clearance, 19.1 L/h/25 kg; volume of the central compartment, 8.5 L/25 kg; volume of the peripheral compartment, 5.8 L/25 kg; and intercompartmental clearance 12.4 L/h/25 kg. Residual unexplained variability was low, at 12.5%. Results suggest the empiric weight-based dosing (mg/kg) used in children <12 kg or 2 years of age may result in subtherapeutic exposure. Model-based dosing of melphalan in pediatric HCT may help inform individualized dosing strategies to improve clinical outcomes and limit drug-related adverse events in pediatric HCT recipients.

Association Between the Magnitude of Intravenous Busulfan Exposure and Development of Hepatic Veno-Occlusive Disease in Children and Young Adults Undergoing Myeloablative Allogeneic Hematopoietic Cell Transplantation.

Intravenous busulfan is widely used as part of myeloablative conditioning regimens in children and young adults undergoing allogeneic hematopoietic cell transplantation (HCT). Hepatic veno-occlusive disease/sinusoidal obstruction syndrome (VOD/SOS) is a serious clinical problem observed with busulfan-based conditioning HCT. The development of VOD/SOS may be associated with busulfan exposure. Getting more insight into the association between busulfan exposure and the development of VOD/SOS enables further optimization of dosing and treatment strategies. The objective of this study was to assess the association between the magnitude of busulfan exposure and the occurrence of VOD/SOS in children and young adults undergoing myeloablative conditioning with a busulfan-containing regimen before allogeneic HCT. In this observational study we included all patients who underwent allogeneic HCT with intravenous busulfan as part of the conditioning regimen at 15 pediatric transplantation centers between 2000 and 2015. The endpoint was the development of VOD/SOS. The magnitude of busulfan exposure was estimated using nonlinear mixed effect modeling and expressed as the maximal concentration (Cmax; day 1 and day 1 to 4 Cmax), cumulative area under the curve (AUC; day 1, highest 1-day AUC in 4 days, and 4-day cumulative AUC), cumulative time above a concentration of 300 µg/L, and clearance on day 1. A total of 88 out of 697 patients (12.6%) developed VOD/SOS. The number of alkylators in the conditioning regimen was a strong effect modifier; therefore we stratified the regression analysis for the number of alkylators. For patients receiving only busulfan as one alkylator (36.3%, n = 253), cumulative busulfan exposure (>78 mg × h/L) was associated with increased VOD/SOS risk (12.6% versus 4.7%; odds ratio [OR] = 2.95, 95% confidence interval [CI] 1.13 to 7.66). For individuals receiving busulfan with one or two additional alkylators (63.7%, n = 444), cumulative busulfan exposure (≤78 and >78 mg × h/L) did not further increase the risk of VOD/SOS (15.4% versus 15.2%; OR = 1.03, 95% CI 0.61 to 1.75). The effect of the magnitude of busulfan exposure on VOD/SOS risk in children and young adults undergoing HCT is dependent on the number of alkylators. In patients receiving busulfan as the only alkylator, higher cumulative busulfan exposure increased the risk of VOD/SOS, whereas in those receiving multiple alkylators, the magnitude of busulfan exposure did not further increase this risk.

Busulfan dose Recommendation in Inherited Metabolic Disorders: Population Pharmacokinetic Analysis.

Busulfan is a commonly used alkylating agent in the conditioning regimens of hematopoietic cell transplantation (HCT). Population pharmacokinetic (popPK) models enable description of busulfan PK and optimization of exposure, which leads to improvement of event-free survival after HCT. Prior busulfan popPK analysis has been limited by small numbers in patients with inherited metabolic disorders (IMD). The primary objective was to characterize population PK of busulfan in a large cohort of children and young adults undergoing HCT for IMD. PopPK analysis of busulfan drug concentrations was performed using data from 78 patients with IMD who received intravenous busulfan (every 24 hours, 4 doses) as part of pretransplantation combination chemotherapy. The final model for busulfan drug clearance was used to estimate individual doses aimed to achieve a target cumulative area under the curve (cAUC) of 80 to 100 mg · h/L. We then compared the probability of cAUC within the range of 80 to 100 mg · h/L by the developed dosing regimen versus conventional regimen. A 1-compartment, linear elimination model best described the PK of busulfan. Significant covariates demonstrated to affect busulfan clearance included total body weight and the time (in days) from busulfan infusion start. The probability of target cAUC attainment by the developed dosing versus the conventional dosing were 47% versus 43% for body weight <12 kg, and 48% versus 36% for body weight ≥12 kg. We described population PK of intravenous busulfan in a large IMD cohort. The proposed dosing regimen based on the developed model can improve the target cAUC attainment of busulfan for IMD.

Population Pharmacokinetic Model Development of Tacrolimus in Pediatric and Young Adult Patients Undergoing Hematopoietic Cell Transplantation.

Background: With a notably narrow therapeutic window and wide intra- and interindividual pharmacokinetic (PK) variability, initial weight-based dosing along with routine therapeutic drug monitoring of tacrolimus are employed to optimize its clinical utilization. Both supratherapeutic and subtherapeutic tacrolimus concentrations can result in poor outcomes, thus tacrolimus PK variability is particularly important to consider in the pediatric population given the differences in absorption, distribution, metabolism, and excretion among children of various sizes and at different stages of development. The primary goals of the current study were to develop a population PK (PopPK) model for tacrolimus IV continuous infusion in the pediatric and young adult hematopoietic cell transplant (HCT) population and implement the PopPK model in a clinically available Bayesian forecasting tool. Methods: A retrospective chart review was conducted of 111 pediatric and young adult patients who received IV tacrolimus by continuous infusion early in the post-transplant period during HCT from February 2016 to July 2020 at our institution. PopPK model building was performed in NONMEM. The PopPK model building process included identifying structural and random effects models that best fit the data and then identifying which patient-specific covariates (if any) further improved model fit. Results: A total of 1,648 tacrolimus plasma steady-state trough concentrations were included in the PopPK modeling process. A 2-compartment structural model best fit the data. Allometrically-scaled weight was a covariate that improved estimation of both clearance and volume of distribution. Overall, model predictions only showed moderate bias, with minor under-prediction at lower concentrations and minor over-prediction at higher predicted concentrations. The model was implemented in a Bayesian dosing tool and made available at the point-of-care. Discussion: Novel therapeutic drug monitoring strategies for tacrolimus within the pediatric and young adult HCT population are necessary to reduce toxicity and improve efficacy in clinical practice. The model developed presents clinical utility in optimizing the use of tacrolimus by enabling model-guided, individualized dosing of IV, continuous tacrolimus via a Bayesian forecasting platform.

5-Azacytidine depletes HSCs and synergizes with an anti-CD117 antibody to augment donor engraftment in immunocompetent mice.

Depletion of hematopoietic stem cells (HSCs) is used therapeutically in many malignant and nonmalignant blood disorders in the setting of a hematopoietic cell transplantation (HCT) to eradicate diseased HSCs, thus allowing donor HSCs to engraft. Current treatments to eliminate HSCs rely on modalities that cause DNA strand breakage (ie, alkylators, radiation) resulting in multiple short-term and long-term toxicities and sometimes even death. These risks have severely limited the use of HCT to patients with few to no comorbidities and excluded many others with diseases that could be cured with an HCT. 5-Azacytidine (AZA) is a widely used hypomethylating agent that is thought to preferentially target leukemic cells in myeloid malignancies. Here, we reveal a previously unknown effect of AZA on HSCs. We show that AZA induces early HSC proliferation in vivo and exerts a direct cytotoxic effect on proliferating HSCs in vitro. When used to pretreat recipient mice for transplantation, AZA permitted low-level donor HSC engraftment. Moreover, by combining AZA with a monoclonal antibody (mAb) targeting CD117 (c-Kit) (a molecule expressed on HSCs), more robust HSC depletion and substantially higher levels of multilineage donor cell engraftment were achieved in immunocompetent mice. The enhanced effectiveness of this combined regimen correlated with increased apoptotic cell death in hematopoietic stem and progenitor cells. Together, these findings highlight a previously unknown therapeutic mechanism for AZA which may broaden its use in clinical practice. Moreover, the synergy we show between AZA and anti-CD117 mAb is a novel strategy to eradicate abnormal HSCs that can be rapidly tested in the clinical setting.

A Guidance for Concomitant Drug Reconciliation Prior to Allogeneic Hematopoietic Cell Transplantation in Children and Young Adults.

Pediatric diseases treated by allogeneic hematopoietic stem cell transplantation (alloHCT) are complex and associated with significant comorbidities and medication requirements that can complicate the transplant process. It is critical to reconcile pre-transplant concomitant medications (pcon-meds) in the weeks prior to alloHCT and to consider the potential for pcon-meds to cause harmful drug-drug interactions (DDIs) or overlapping toxicities with conditioning agents. In this perspective, we describe a systematic process to review pcon-meds and determine the drug modifications needed to avoid DDIs with conditioning regimens. We provide an extensive appendix with timelines for discontinuation or modification of common pcon-meds that patients are taking when presenting to the HCT medical team. The timelines are based on the pharmacokinetic (PK) properties of both the pcon-meds and the planned conditioning medications, as well as anticipated DDIs. They also account for the ages seen at pediatric transplant centers (0-30 years old). Common scenarios, such as when pcon-med discontinuation is not an option, are discussed. Since alloHCT patients are often dependent upon psychiatric medications with problematic DDIs, a table of alternative, non-interacting psychiatric medications is also presented. The appendix provides details regarding how to adjust pcon-meds prior to the start of chemotherapy for children and young adults undergoing alloHCT, however patient-specific circumstances always need to be taken into account. Careful attentiveness to pcon-meds at the time the decision is made to pursue transplant will result in more consistent HCT outcomes, with lower toxicity and increased efficacy of conditioning agents.

Transplant-associated thrombotic microangiopathy in pediatric patients: pre-HSCT risk stratification and prophylaxis.

Transplant-associated thrombotic microangiopathy (TA-TMA) is an endothelial injury syndrome that complicates hematopoietic stem cell transplant (HSCT). Morbidity and mortality from TA-TMA remain high, making prevention critical. We describe our retrospective single-center experience of TA-TMA after pediatric allogeneic HSCT and present a novel pre-HSCT risk-stratification system and prophylaxis regimen. From January 2012 through October 2019, 257 patients underwent 292 allogeneic HSCTs. Prospective risk stratification was introduced in December 2016. High-risk (HR) patients were treated with combination prophylaxis with eicosapentaenoic acid and N-acetylcysteine. The 1-year cumulative incidence of TA-TMA was 6.3% (95% confidence interval [CI], 3.2-9.4). Age ≥10 years, myeloablative conditioning with total body irradiation, HLA mismatch, diagnosis of severe aplastic anemia or malignancy, prior calcineurin inhibitor exposure, and recipient cytomegalovirus seropositivity were found to be pre-HSCT risk factors for development of TA-TMA. Before routine prophylaxis, TA-TMA rates were significantly different between the HR and standard-risk groups, at 28.2% (95% CI, 0-12.7) vs 3.2% (0.1-6.3), respectively (P < .001). After introduction of prophylaxis, the 1-year cumulative incidence of TA-TMA in the HR group decreased to 4.5% (95% CI, 0-13.1; P = .062, compared with the incidence before prophylaxis). Multicenter pediatric studies are needed to validate these risk criteria and to confirm the efficacy of the prophylactic regimen.

The Relationship Between Busulfan Exposure and Achievement of Sustained Donor Myeloid Chimerism in Patients with Non-Malignant Disorders.

The overall objective of allogeneic hematopoietic cell transplantation (HCT) in patients with non-malignant conditions involves replacing a dysfunctional or absent cell or gene product for disease correction. It is unclear whether lower busulfan exposure may be sufficient in this population to facilitate durable myeloid engraftment and limit toxicity. Given that neither the ideal level of mixed myeloid chimerism for specific non-malignant diseases nor how to condition a patient to achieve stable mixed myeloid chimerism is fully known, we sought to analyze the relationships among busulfan exposure, myeloid chimerism, and outcomes in patients with non-malignant conditions receiving busulfan as a part of combination pretransplant conditioning at our institution. This was a single-center, retrospective study including pediatric patients with a variety of non-malignant disorders who underwent allogeneic HCT at the University of California San Francisco Benioff Children's Hospital from March 2007 to June 2018. The busulfan cumulative area under the curve (cAUC) was estimated using a validated population pharmacokinetic model and nonlinear mixed effects modeling. Median busulfan cAUC for all patients was 70 mg·h/L (range, 53 to 108). All of the 29 patients with a busulfan cAUC of ≥70 mg·h/L achieved long-term disease correction with full or stable mixed (>20%) myeloid chimerism, compared to 78.5% (22/28) of patients with a cAUC of <70 mg·h/L (P = .01). Overall ksurvival was evaluated up to 3 years and was identical in patients with busulfan cAUC < 70 mg·h/L and patients with busulfan cAUC ≥70 mg·h/L (96% versus 93%; P = .92). Only three patients died, at days 65, 164 and 980 days post-HCT. Severe busulfan-related toxicities and graft-versus-host-disease (GVHD) were rare, with veno-occlusive disease occurring in four patients (7%), acute respiratory distress syndrome in three patients (5%), and GVHD in five patients (9%). These results demonstrate excellent outcomes and extremely low rates of toxicity across our entire cohort. Based on the results of this study, we recommend a busulfan exposure target of 75 mg·h/L (range, 70 to 80) in all non-malignant patients receiving allogeneic HCT to ensure optimal exposure for achievement of high-level stable myeloid chimerism.

Prescribing Prevalence of Medications With Potential Genotype-Guided Dosing in Pediatric Patients.

Importance: Genotype-guided prescribing in pediatrics could prevent adverse drug reactions and improve therapeutic response. Clinical pharmacogenetic implementation guidelines are available for many medications commonly prescribed to children. Frequencies of medication prescription and actionable genotypes (genotypes where a prescribing change may be indicated) inform the potential value of pharmacogenetic implementation. Objective: To assess potential opportunities for genotype-guided prescribing in pediatric populations among multiple health systems by examining the prevalence of prescriptions for each drug with the highest level of evidence (Clinical Pharmacogenetics Implementation Consortium level A) and estimating the prevalence of potential actionable prescribing decisions. Design, Setting, and Participants: This serial cross-sectional study of prescribing prevalences in 16 health systems included electronic health records data from pediatric inpatient and outpatient encounters from January 1, 2011, to December 31, 2017. The health systems included academic medical centers with free-standing children's hospitals and community hospitals that were part of an adult health care system. Participants included approximately 2.9 million patients younger than 21 years observed per year. Data were analyzed from June 5, 2018, to April 14, 2020. Exposures: Prescription of 38 level A medications based on electronic health records. Main Outcomes and Measures: Annual prevalence of level A medication prescribing and estimated actionable exposures, calculated by combining estimated site-year prevalences across sites with each site weighted equally. Results: Data from approximately 2.9 million pediatric patients (median age, 8 [interquartile range, 2-16] years; 50.7% female, 62.3% White) were analyzed for a typical calendar year. The annual prescribing prevalence of at least 1 level A drug ranged from 7987 to 10 629 per 100 000 patients with increasing trends from 2011 to 2014. The most prescribed level A drug was the antiemetic ondansetron (annual prevalence of exposure, 8107 [95% CI, 8077-8137] per 100 000 children). Among commonly prescribed opioids, annual prevalence per 100 000 patients was 295 (95% CI, 273-317) for tramadol, 571 (95% CI, 557-586) for codeine, and 2116 (95% CI, 2097-2135) for oxycodone. The antidepressants citalopram, escitalopram, and amitriptyline were also commonly prescribed (annual prevalence, approximately 250 per 100 000 patients for each). Estimated prevalences of actionable exposures were highest for oxycodone and ondansetron (>300 per 100 000 patients annually). CYP2D6 and CYP2C19 substrates were more frequently prescribed than medications influenced by other genes. Conclusions and Relevance: These findings suggest that opportunities for pharmacogenetic implementation among pediatric patients in the US are abundant. As expected, the greatest opportunity exists with implementing CYP2D6 and CYP2C19 pharmacogenetic guidance for commonly prescribed antiemetics, analgesics, and antidepressants.

Lower Exposure to Busulfan Allows for Stable Engraftment of Donor Hematopoietic Stem Cells in Children with Mucopolysaccharidosis Type I: A Case Report of Four Patients.

Busulfan is an alkylating agent routinely used in conditioning regimens prior to allogeneic hematopoietic cell transplantation (HCT) for various nonmalignant disorders, including inborn errors of metabolism. The combination of model-based dosing and therapeutic drug monitoring (TDM) of busulfan pharmacokinetics (PK) to a lower exposure target has the potential to reduce the regimen-related toxicity while opening marrow niches sufficient for engraftment in diseases such as mucopolysaccharidosis type I (MPS I). We present four cases of the severe form of MPS I or Hurler syndrome, demonstrating successful and stable CD14/15 donor chimerism following the prospective application of model-based dosing and TDM aimed to achieve lower busulfan exposure. All patients received a busulfan-based conditioning regimen with a median cumulative area-under-the-curve (cAUC) target of 63.7 mg h/L (range, 62.4 to 65.0) in protocol-specific combination of chemotherapeutic regimen. The donor source was unrelated umbilical cord blood for three patients and matched sibling donor bone marrow for one patient. The observed median busulfan cAUC was 66.1 mg h/L (range, 65.2 to 70.6) and was within 10% of the intended target. Stable, full donor myeloid chimerism was achieved for three patients, while one patient achieved a stable mixed chimerism (76% donor CD14/15 at 53 months) without a recurring need for enzyme replacement. The normalization of α-L-iduronidase enzyme levels followed the attainment of successful donor myeloid chimerism in all patients. Regimen-related toxicity remained low with no evidence of acute graft-versus-host disease (GVHD) grades II to IV and chronic GVHD.