Automated Combustion Accelerator Mass Spectrometry for the Analysis of Biomedical Samples in the Low Attomole Range

The development and evolution of biological AMS at Livermore: a perspective

Biological accelerator mass spectrometry (AMS) provides ultrasensitive carbon-14 isotopic analysis enabling a deeper understanding of human health concerns by enabling quantification of pharmacokinetics and other molecular endpoints directly in humans. It enables environmentally and human relevant studies of metabolic pathways through the use of very low concentrations of labeled metabolic substrates in cells and organisms. Here, we discuss why AMS is an important tool for the biosciences, the development and evolution of biological AMS at Livermore and discuss technical refinements that will improve the efficiency of operation for the measurement of ultra-trace levels of 14C, which, long term, will enable greater ease of use and sample throughput.

The Pharmacokinetics, Metabolism, and Clearance Mechanisms of Ritlecitinib, a Janus Kinase 3 and Tyrosine-Protein Kinase Family Inhibitor, in Humans

Ritlecitinib is an oral once-daily irreversible inhibitor of Janus kinase 3 and tyrosine-protein kinase family being developed for the treatment of moderate-to-severe alopecia areata. This study examined the disposition of ritlecitinib in male participants following oral and intravenous administration using accelerator mass spectroscopy methodology to estimate pharmacokinetic parameters and characterize metabolite profiles. The results indicated ritlecitinib had a systemic clearance of 43.7 L/h, a steady state volume of distribution of 73.8 L, extent of absorption of 89%, time to maximum plasma concentration of ∼0.5 hours, and absolute oral bioavailability of 64%. An observed long terminal half-life of total radioactivity was primarily attributed to ritlecitinib binding to plasma albumin. Ritlecitinib was the main circulating drug species in plasma (∼30%), with one major pharmacologically inactive cysteine conjugated metabolite (M2) at >10%. Oxidative metabolism (fractional clearance 0.47) and glutathione-related conjugation (fractional clearance 0.24) were the primary routes of elimination for ritlecitinib with the greatest disposition of radioactivity shown in the urine (∼71%). In vitro phenotyping indicated ritlecitinib cytochrome P450 (CYP) fraction of metabolism assignments of 0.29 for CYP3A, 0.09 for CYP2C8, 0.07 for CYP1A2, and 0.02 for CYP2C9. In vitro phenotyping in recombinant human glutathione S-transferases indicated ritlecitinib was turned over by a number of cytosolic and microsomal enzyme isoforms. SIGNIFICANCE STATEMENT: This study provides a detailed understanding of the disposition and metabolism of ritlecitinib, a JAK3 and TEC family kinase inhibitor for alopecia areata in humans, as well as characterization of clearance pathways and pharmacokinetics of ritlecitinib and its metabolites. As an AMS-based ADME study design, we have expanded on reporting the standard ADME endpoints, providing key pharmacokinetic parameters, such as clearance, volume of distribution, and bioavailability, allowing for a more comprehensive understanding of drug disposition.

A general perspective for the conduct of radiolabelled distribution, metabolism, and excretion studies for antibody-drug conjugates

Antibody-drug conjugates (ADCs) are a class of biopharmaceuticals that combine the specificity of monoclonal antibodies (mAbs) with the cytotoxicity of small molecule drugs. 15 ADCs have been approved by regulatory authorities up to now, mainly for indications in oncology, however, this review paper will only focus on the 13 ADCs that have been approved by either the FDA or EMA.ADME (Absorption, Distribution, Metabolism, and Excretion) studies are essential for the development of small molecule drugs to evaluate their disposition properties. These studies help to select drug candidates, determine the optimal dosing regimen and help to identify potential safety concerns for the drug of interest in human. Tissue distribution studies are also important as they facilitate the understanding of the efficacy and safety for parent drug and its metabolites in preclinical and clinical studies.For biologics, ADME studies are usually not required. In this paper, we review the existing approval packages and literature for approved ADCs to determine the extent of ADME studies performed as part of ADC registration packages.We conclude that ADME studies are recommended for the development of ADCs if new linkers and payloads are used that have never been used in humans before as these studies provide valuable information on the pharmacokinetic properties, optimal dosing regimen, and potential safety concerns. However, for the development of ADCs with established linker payload combinations, radiolabelled ADME studies may not be necessary if the distribution, metabolism and excretion properties have been described before. Clinical radiolabelled ADME studies are not recommended where patients are treated for life threating diseases like for indications in oncology.

A Phase I, Open-Label, Mass Balance Study of [14C]-Iberdomide in Healthy Subjects

BACKGROUND

Iberdomide is a novel potent cereblon modulator (CELMoD®) agent, which is currently under clinical development for hematological malignancies. A human mass balance study was conducted to characterize the biotransformation and excretion pathways of iberdomide.

METHOD

After a single dose of radiolabelled [14C]-iberdomide (1 mg) in six healthy subjects. Blood, urine, and fecal samples were collected for pharmacokinetics, mass balance, and clinical laboratory assessments.

RESULTS

Results showed that a single oral dose of 1 mg iberdomide was generally well tolerated in healthy subjects. The recovery of [14C]-iberdomide-derived radioactivity in humans was 45.9% in urine and 42.6% in feces. Based on exposure (area under the concentration-time curve [AUC0-24]), iberdomide and M12 (metabolites) accounted for approximately 59% and 14% of circulating total radioactivity (TRA) exposure, respectively. Of the 88.5% TRA excreted, approximately 27% was excreted as unchanged iberdomide and 62% as metabolites, with similar amounts of excreted metabolites in the urine (16%) and feces (11%).

CONCLUSION

Biotransformation of iberdomide in humans included multiple oxidations of the morpholino moiety as well as glutarimide ring hydrolysis of parent and oxidized metabolites and a combination of these pathways. Iberdomide was the predominant component in human plasma, with metabolite M12 being the most prominent circulating metabolite. In excreta, similar iberdomide-derived radioactivity was found in urine and feces.

TRIAL REGISTRATION NUMBER

NCT03294603.

The Metabolism of Lufotrelvir, a Prodrug Investigated for the Treatment of SARS-COV2 in Humans Following Intravenous Administration

The metabolism of lufotrelvir, a novel phosphate prodrug of PF-00835231 for the treatment of COVID-19, was evaluated in healthy human volunteers and clinical trial participants with COVID-19 following intravenous infusion. The prodrug was completely converted to PF-00835231 that was subsequently cleared by hydrolysis, hydroxylation, ketoreduction, epimerization, renal clearance, and secretion into the feces. The main circulating metabolite was a hydrolysis product (M7) that was present at concentrations greater than PF-00835231, and this was consistent between healthy volunteers and participants with COVID-19. On administration of [14C]lufotrelvir, only 63% of the dose was obtained in excreta over 10 days and total drug-related material demonstrated a prolonged terminal phase half-life in plasma. A considerable portion of the labeled material was unextractable from fecal homogenate and plasma. The position of the carbon-14 atom in the labeled material was at a leucine carbonyl, and pronase digestion of the pellet derived from extraction of the fecal homogenate showed that [14C]leucine was released. SIGNIFICANCE STATEMENT: Lufotrelvir is an experimental phosphate prodrug intravenous therapy investigated for the potential treatment of COVID-19 in a hospital setting. The overall metabolism of lufotrelvir was determined in human healthy volunteers and clinical trial participants with COVID-19. Conversion of the phosphate prodrug to the active drug PF-00835231 was complete and the subsequent metabolic clearance of the active drug was largely via amide bond hydrolysis. Substantial drug-related material was not recovered due to loss of the carbon-14 label to endogenous metabolism.

A phase 1 study to investigate the absorption, distribution, metabolism and excretion of brepocitinib in healthy males using a 14 C-microdose approach

AIMS

Brepocitinib is a tyrosine kinase 2/Janus kinase 1 inhibitor being investigated for the treatment of several autoimmune diseases. This study assessed the absorption, distribution, metabolism and excretion of oral brepocitinib, and the absolute oral bioavailability (F) and fraction absorbed (Fa ) using a 14 C microtracer approach.

METHODS

This was a phase 1 open-label, nonrandomized, fixed sequence, two-period, single-dose study of brepocitinib in healthy male participants. Participants received a single oral 60 mg dose of 14 C brepocitinib (~300 nCi) in Period A, then an unlabelled oral 60 mg dose followed by an intravenous (IV) 30 μg dose of 14 C labelled brepocitinib (~300 nCi) in Period B. Mass balance, pharmacokinetic parameters and safety were assessed.

RESULTS

Six participants were enrolled. Brepocitinib was absorbed rapidly following oral administration. In Period A, total recovery of the oral dose was 96.7% ± 6.3% (88.0% ± 8.0% in urine, 8.7% ± 2.1% in faeces). In Period B, a small fraction (6.0% of the oral dose) was recovered unchanged in urine. F and Fa were 74.6% (90% confidence interval 67.3%, 82.8%) and 106.9%, respectively. Brepocitinib demonstrated an acceptable safety profile and was well tolerated following oral or oral then IV administrations. No deaths, serious adverse events or discontinuations were reported.

CONCLUSION

Intestinal absorption of brepocitinib was essentially complete after oral administration, with F ~75%. Drug-related material recovery was high, with the majority excreted in urine. The major route of elimination of brepocitinib was renal excretion as metabolites, whereas urinary elimination of unchanged brepocitinib was minor. NCT: NCT03770039.

Mass Balance and Absorption, Distribution, Metabolism, and Excretion Properties of Balcinrenone following Oral Administration in Combination with Intravenous Microtracer in Healthy Subjects

An absorption, distribution, metabolism, and excretion study was performed to determine the basic pharmacokinetic parameters, mass balance, and metabolite profiles of balcinrenone, a mineralocorticoid receptor modulator, in humans. This open-label, single-center, nonrandomized study had a two-period design. In period 1, eight healthy male subjects were dosed with a microtracer intravenous infusion of [14C]balcinrenone shortly after receiving an oral dose of unlabeled balcinrenone in a capsule. Following a 7-day washout, the same group of subjects subsequently received an oral dose of [14C]balcinrenone as a suspension in period 2. Clearance and absolute bioavailability of balcinrenone were determined to be 14.2 l/h and 52%, respectively. Renal clearance was determined to be 5.4 l/h (>fu • glomerular filtration rate), indicating elimination via active tubular secretion, which was potentially mediated by P-glycoprotein 1 and/or organic anion transporter 3, according to in vitro transporter data. In total, 94.1% of the oral dose was recovered: 45.2% in the urine and 48.9% in the feces. Balcinrenone was primarily metabolized via oxidation, and in vitro data suggest that cytochrome P450 3A4 was the main enzyme responsible. Intact [14C]balcinrenone accounted for 55% of drug-related material in the plasma; four metabolites were identified, each representing <6% of the total plasma radioactivity. In conclusion, this two-period study has determined the basic pharmacokinetic parameters of balcinrenone in humans, including absolute bioavailability and disposition. No metabolites warranted further evaluation on account of their low representation, and any contribution to the pharmacodynamic response or potential drug-drug interactions was deemed negligible. SIGNIFICANCE STATEMENT: This study provides a detailed understanding of the pharmacokinetics, disposition, and metabolism of balcinrenone following oral and microtracer intravenous administration in humans. In vitro phenotyping and transporter data granted mechanistic insights into the absorption, distribution, metabolism, and excretion properties of balcinrenone. This knowledge will guide future nonclinical and clinical studies evaluating drug-drug interactions, organ dysfunction, and safety of metabolites.

Human Absorption, Distribution, Metabolism, and Excretion Studies: Origins, Innovations, and Importance

Human absorption, distribution, metabolism, and excretion (hADME) studies represent one of the most important clinical studies in terms of obtaining a comprehensive and quantitative overview of the total disposition of a drug. This article will provide background on the origins of hADME studies as well as provide an overview of technological innovations that have impacted how hADME studies are carried out and analyzed. An overview of the current state of the art for hADME studies will be provided, the impacts of advances in technology and instrumentation on the timing of and approaches to hADME studies will be discussed, and a summary of the parameters and information obtained from these studies will be offered. Additionally, aspects of the ongoing debate over the importance of animal absorption, distribution, metabolism, and excretion studies versus a "human-first, human-only strategy" will be presented. Along with the information above, this manuscript will highlight how, for over 50 years, Drug Metabolism and Disposition has served as an important outlet for the reporting of hADME studies. SIGNIFICANCE STATEMENT: Human absorption, distribution, metabolism, and excretion (hADME) studies have and will continue to be important to the understanding and development of drugs. This manuscript provides a historical perspective on the origins of hADME studies as well as advancements resulting in the current-state-of the art practice for these studies.

Application of Accelerator Mass Spectrometry to Characterize the Mass Balance Recovery and Disposition of AZD4831, a Novel Myeloperoxidase Inhibitor, following Administration of an Oral Radiolabeled Microtracer Dose in Humans

This study evaluated the mass balance and disposition of AZD4831, a novel myeloperoxidase inhibitor, in six healthy participants using a 14C-labeled microtracer coupled with analysis by accelerator mass spectrometry (AMS). A single oral dose of 10 mg 14C-AZD4831 (14.8 kBq) was administered as a solution, and 14C levels were quantified by AMS in blood, urine, and feces over 336 hours postdose. AZD4831 was rapidly absorbed, and AZD4831 plasma concentrations declined in a biphasic manner, with a long half-life of 52 hours. AZD4831 was eliminated via metabolism and renal excretion. An N-carbamoyl glucuronide metabolite of AZD4831 (M7), formed primarily via UGT1A1, was the predominant circulating metabolite. Presumably, M7 contributed to the long half-life of AZD4831 via biliary elimination and hydrolysis/enterohepatic recirculation of AZD4831. On average, ∼84% of administered 14C-AZD4831 was recovered by 336 hours postdose (urine, 51.2%; feces, 32.4%). Between 32%-44% of the dose was excreted as unchanged AZD4831 in urine, indicating renal elimination as the major excretory route. Only 9.7% of overall fecal recovery was recorded in the first 48 hours, with the remainder excreted over 48%-336 hours, suggesting that most fecal recovery was due to biliary elimination. Furthermore, only 6% of unchanged AZD4831 was recovered in feces. Overall, the fraction of the administered AZD4831 dose absorbed was high. 14C-AZD4831 was well tolerated. These findings contribute to increasing evidence that human absorption, distribution, metabolism, and excretion studies can be performed with acceptable mass balance recovery at therapeutically relevant doses and low radiolabel-specific activity using an AMS-14C microtracer approach. SIGNIFICANCE STATEMENT: In this study, the human absorption, distribution, metabolism, and excretion (hADME) of the novel myeloperoxidase inhibitor AZD4831 was assessed following oral administration. This included investigation of the disposition of M7, the N-carbamoyl glucuronide metabolite. Resolution of challenges highlighted in this study contributes to increasing evidence that hADME objectives can be achieved in a single study for compounds with therapeutically relevant doses and low radiolabel-specific activity by using an AMS-14C microtracer approach, thus reducing the need for preclinical radiolabeled studies.

Considerations for Human ADME Strategy and Design Paradigm Shift(s) - An Industry White Paper

The human absorption, distribution, metabolism, and excretion (hADME) study is the cornerstone of the clinical pharmacology package for small molecule drugs, providing comprehensive information on the rates and routes of disposition and elimination of drug-related material in humans through the use of 14 C-labeled drug. Significant changes have already been made in the design of the hADME study for many companies, but opportunity exists to continue to re-think both the design and timing of the hADME study in light of the potential offered by newer technologies, that enable flexibility in particular to reducing the magnitude of the radioactive dose used. This paper provides considerations on the variety of current strategies that exist across a number of pharmaceutical companies and on some of the ongoing debates around a potential move to the so called "human first/human only" approach, already adopted by at least one company. The paper also provides a framework for continuing the discussion in the application of further shifts in the paradigm.

HPLC-Parallel accelerator and molecular mass spectrometry analysis of 14C-labeled amino acids

Accelerator mass spectrometry (AMS) is the method of choice for quantitation of low amounts of 14C-labeled biomolecules. Despite exquisite sensitivity, an important limitation of AMS is its inability to provide structural information about the analyte. This limitation is not critical when the labeled compounds are well-characterized prior to AMS analysis. However, analyte identity is important in other experiments where, for example, a compound is metabolized and the structures of its metabolites are not known. We previously described a moving wire interface that enables direct AMS measurement of liquid sample in the form of discrete drops or HPLC eluent without the need for individual fraction collection, termed liquid sample-AMS (LS-AMS). We now report the coupling of LS-AMS with a molecular mass spectrometer, providing parallel accelerator and molecular mass spectrometry (PAMMS) detection of analytes separated by liquid chromatography. The repeatability of the method was examined by performing repeated injections of 14C-labeled tryptophan, and relative standard deviations of the 14C peak areas were ≤10.57% after applying a normalization factor based on a standard. Five 14C-labeled amino acids were separated and detected to provide simultaneous quantitative AMS and structural MS data, and AMS results were compared with solid sample-AMS (SS-AMS) data using Bland-Altman plots. To demonstrate the utility of the workflow, yeast cells were grown in a medium with 14C-labeled tryptophan. The cell extracts were analyzed by PAMMS, and 14C was detected in tryptophan and its metabolite kynurenine.

The Pharmacokinetics, Metabolism, and Clearance Mechanisms of Abrocitinib, a Selective Janus Kinase Inhibitor, in Humans

Abrocitinib is an oral once-daily Janus kinase 1 selective inhibitor being developed for the treatment of moderate-to-severe atopic dermatitis. This study examined the disposition of abrocitinib in male participants following oral and intravenous administration using accelerator mass spectroscopy methodology to estimate pharmacokinetic parameters and characterize metabolite (M) profiles. The results indicated abrocitinib had a systemic clearance of 64.2 L/h, a steady-state volume of distribution of 100 L, extent of absorption >90%, time to maximum plasma concentration of ∼0.5 hours, and absolute oral bioavailability of 60%. The half-life of both abrocitinib and total radioactivity was similar, with no indication of metabolite accumulation. Abrocitinib was the main circulating drug species in plasma (∼26%), with 3 major monohydroxylated metabolites (M1, M2, and M4) at >10%. Oxidative metabolism was the primary route of elimination for abrocitinib, with the greatest disposition of radioactivity shown in the urine (∼85%). In vitro phenotyping indicated abrocitinib cytochrome P450 fraction of metabolism assignments of 0.53 for CYP2C19, 0.30 for CYP2C9, 0.11 for CYP3A4, and ∼0.06 for CYP2B6. The principal systemic metabolites M1, M2, and M4 were primarily cleared renally. Abrocitinib, M1, and M2 showed pharmacology with similar Janus kinase 1 selectivity, whereas M4 was inactive. SIGNIFICANCE STATEMENT: This study provides a detailed understanding of the disposition and metabolism of abrocitinib, a Janus kinase inhibitor for atopic dermatitis, in humans, as well as characterization of clearance pathways and pharmacokinetics of abrocitinib and its metabolites.

A Phase 1 Study to Assess Mass Balance and Absolute Bioavailability of Zimlovisertib in Healthy Male Participants Using a 14 C-Microtracer Approach

Zimlovisertib (PF‐06650833) is a selective, reversible inhibitor of interleukin‐1 receptor‐associated kinase 4 (IRAK4) with anti‐inflammatory effects. This phase 1, open‐label, fixed‐sequence, two‐period, single‐dose study aimed to evaluate the mass balance and excretion rate of zimlovisertib in healthy male participants using a ¹⁴C‐microtracer approach. All six participants received 300 mg ¹⁴C‐zimlovisertib with lower radioactivity per mass unit orally in Period A, then unlabeled zimlovisertib 300 mg orally and ¹⁴C‐zimlovisertib 135 μg intravenously (IV) in Period B. Study objectives included extent and rate of excretion of ¹⁴C‐zimlovisertib, pharmacokinetics, and safety and tolerability of oral and IV zimlovisertib. Total radioactivity recovered in urine and feces was 82.4% ± 6.8% (urine 23.1% ± 12.3%, feces 59.3% ± 9.7%) in Period A. Zimlovisertib was absorbed rapidly following oral administration, with the fraction absorbed estimated to be 44%. Absolute oral bioavailability of the 300‐mg dose was 17.4% (90% confidence interval 14.1%, 21.5%) using the dose‐normalized area under the concentration–time curve from time 0 to infinity. There were no deaths, serious adverse events (AEs), severe AEs, discontinuations or dose reductions due to AEs, and no clinically significant laboratory abnormalities. These results demonstrate that zimlovisertib had low absolute oral bioavailability and low absorption (<50%).

Microdosing as a Potential Tool to Enhance Clinical Development of Novel Antibiotics: A Tissue and Plasma PK Feasibility Study with Ciprofloxacin

BACKGROUND AND OBJECTIVE

In microdose studies, drug pharmacokinetics is measured in humans after administration of subtherapeutic doses. While previous microdose studies focused primarily on plasma pharmacokinetics, we set out to evaluate the feasibility of microdosing for a pharmacokinetic assessment in subcutaneous tissue and epithelial lining fluid.

METHODS

Healthy subjects received a single intravenous bolus injection of a microdose of [14C]ciprofloxacin (1.1 µg, 7 kBq) with (cohort A, n = 9) or without (cohort B, n = 9) a prior intravenous infusion of a therapeutic dose of unlabeled ciprofloxacin (400 mg). Microdialysis and bronchoalveolar lavage were applied for determination of subcutaneous and intrapulmonary drug concentrations. Microdose [14C]ciprofloxacin was quantified by accelerator mass spectrometry and therapeutic-dose ciprofloxacin by liquid chromatography-tandem mass spectrometry.

RESULTS

The pharmacokinetics of therapeutic-dose ciprofloxacin (cohort A) in plasma, subcutaneous tissue, and epithelial lining fluid was in accordance with previous data. In plasma and subcutaneous tissue, the dose-adjusted area under the concentration-time curve of microdose ciprofloxacin was similar in cohorts A and B and within an 0.8-fold to 1.1-fold range of the area under the concentration-time curve of therapeutic-dose ciprofloxacin. Penetration of microdose ciprofloxacin into subcutaneous tissue was similar in cohorts A and B and comparable to that of therapeutic-dose ciprofloxacin with subcutaneous tissue-to-plasma area under the concentration-time curve ratios of 0.44, 0.44, and 0.38, respectively. Penetration of microdose ciprofloxacin into epithelial lining fluid was highly variable and failed to predict the epithelial lining fluid penetration of therapeutic-dose ciprofloxacin.

CONCLUSIONS

Our study confirms the feasibility of microdosing for pharmacokinetic measurements in plasma and subcutaneous tissue. Microdosing combined with microdialysis is a potentially useful tool in clinical antimicrobial drug development, but its applicability for the assessment of pulmonary pharmacokinetics with bronchoalveolar lavage requires further studies.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov NCT03177720 (registered 6 June, 2017).

Quantification of azacitidine incorporation into human DNA/RNA by accelerator mass spectrometry as direct measure of target engagement

We report an accelerator mass spectrometry (AMS) assay to quantify azacitidine (Aza) incorporation into DNA and RNA from human acute myeloid leukemia (AML) cells, mouse bone marrow (BM) and peripheral blood mononuclear cells (PBMCs). Aza, a cytidine nucleoside analogue, is a disease modifying pharmacological agent used for treatment of myelodysplastic syndromes (MDS) and AML. Our assay was able to directly quantify the complex of Aza incorporated into DNA/RNA, via isolation of DNA/RNA from matrix (i.e., cancer cells, BM and PBMC) and subsequent measurement of total radioactivity (i.e., ¹⁴C-Aza) by using AMS. The sensitivity of the method was able to quantify as little as a single Aza molecule incorporated into DNA with approximately 2 × 10⁷ nucleotides from PBMCs. An in vivo mouse model was used for establishing the lower limits of quantification (LLOQs) for Aza incorporated into DNA/RNA in mouse PBMCs (∼ 3.7 × 10⁵) and BM (∼27.8 mg) collected 24 h post-dose after total exposure of 18 nCi/mouse (Aza 1 mg/kg). The LLOQs for PBMC analysis were 2.5 picogram equivalents per microgram (pgEq/μg) DNA and 0.22 pgEq/μg RNA, and for BM analysis were 1.7 pgEq/μg DNA and 0.22 pgEq/μg RNA. A linear relationship (i.e., ∼10-fold) was established of radioactive dose from ¹⁴C-Aza 17 nCi/mouse to 188 nCi/mouse and AMS response (i.e., ¹⁴C/¹²C ratio ranging from 2.45 × 10^-11 to 2.50 × 10^-10), as Aza was incorporated into DNA in mouse BM. The current method enables the direct measurement of Aza incorporation into DNA and RNA from patient PBMCs and BM to provide dosing optimization, and to assess target engagement with as little as ∼5 mL whole blood and ∼3 mL of BM from patients.

Double Trap Interface: A novel gas interface for high throughput analysis of biomedical samples by AMS

Although Accelerator Mass Spectrometry (AMS) offers unparalleled sensitivity by investigating the fate of ¹⁴C-labeled compounds within the organism, its widespread use in ADME (absorption, distribution, metabolism, excretion) studies is limited. Conventional approaches based on Liquid Scintillation Counting (LSC) are still preferred, in particular because of complexity and costs associated with AMS measurements. Progress made over the last decade towards more compact AMS systems increased the interest in a combustion-based AMS approach allowing the analysis of samples in gaseous form. Thus, a novel gas Double Trap Interface (DTI) was designed, providing high sample throughput for the analysis of biomedical samples. DTI allows the coupling of an Elemental Analyzer (EA) for sample combustion to the hybrid ion source of a MICADAS (MIni CArbon DAting System) AMS system. The performance was evaluated in two studies through the analysis of more than 1000 samples from ¹⁴C-labeled biomatrices and fractions collected after liquid chromatography (LC). The covered activity ranged from 1 to 1000 mBq/g for labeled biomatrices and from 1 to 10000 mBq/g(C) for LC fractions. The implemented routine allows automated measurements requiring less than 5 min per sample (12-13 analyses per hour) without the need for sample conversion to graphite.

Mass balance and pharmacokinetics of an oral dose of 14 C-napabucasin in healthy adult male subjects

This phase 1, open-label study assessed14 C-napabucasin absorption, metabolism, and excretion, napabucasin pharmacokinetics, and napabucasin metabolites (primary objectives); safety/tolerability were also evaluated. Eight healthy males (18-45 years) received a single oral 240-mg napabucasin dose containing ~100 μCi14 C-napabucasin. Napabucasin was absorbed and metabolized to dihydro-napabucasin (M1; an active metabolite [12.57-fold less activity than napabucasin]), the sole major circulating metabolite (median time to peak concentration: 2.75 and 2.25 h, respectively). M1 plasma concentration versus time profiles generally mirrored napabucasin; similar arithmetic mean half-lives (7.14 and 7.92 h, respectively) suggest M1 formation was rate limiting. Napabucasin systemic exposure (per Cmax and AUC) was higher than M1. The total radioactivity (TRA) whole blood:plasma ratio (AUClast : 0.376; Cmax : 0.525) indicated circulating drug-related compounds were essentially confined to plasma. Mean TRA recovery was 81.1% (feces, 57.2%; urine, 23.8%; expired air, negligible). Unlabeled napabucasin and M1 recovered in urine accounted for 13.9% and 11.0% of the dose (sum similar to urine TRA recovered); apparent renal clearance was 8.24 and 7.98 L/h. No uniquely human or disproportionate metabolite was quantified. Secondary glucuronide and sulfate conjugates were common urinary metabolites, suggesting napabucasin was mainly cleared by reductive metabolism. All subjects experienced mild treatment-emergent adverse events (TEAEs), the majority related to napabucasin. The most commonly reported TEAEs were gastrointestinal disorders. There were no clinically significant laboratory, vital sign, electrocardiogram, or physical examination changes. Napabucasin was absorbed, metabolized to M1 as the sole major circulating metabolite, and primarily excreted via feces. A single oral 240-mg dose was generally well tolerated.

The Oral Bioavailability and Metabolism of Midazolam in Stable Critically Ill Children: A Pharmacokinetic Microtracing Study

Midazolam is metabolized by the developmentally regulated intestinal and hepatic drug-metabolizing enzyme cytochrome P450 (CYP) 3A4/5. It is frequently administered orally to children, yet knowledge is lacking on the oral bioavailability in term neonates up until 1 year of age. Furthermore, the dispositions of the major metabolites 1-OH-midazolam (OHM) and 1-OH-midazolam-glucuronide (OHMG) after oral administration are largely unknown for the entire pediatric age span. We aimed to fill these knowledge gaps with a pediatric [14 C]midazolam microtracer population pharmacokinetic study. Forty-six stable, critically ill children (median age 9.8 (range 0.3-276.4) weeks) received a single oral [14 C]midazolam microtracer (58 (40-67) Bq/kg) when they received a therapeutic continuous intravenous midazolam infusion and had an arterial line in place enabling blood sampling. For midazolam, in a one-compartment model, bodyweight was a significant predictor for clearance (0.98 L/hour) and volume of distribution (8.7 L) (values for a typical individual of 5 kg). The typical oral bioavailability in the population was 66% (range 25-85%). The exposures of OHM and OHMG were highest for the youngest age groups and significantly decreased with postnatal age. The oral bioavailability of midazolam, largely reflective of intestinal and hepatic CYP3A activity, was on average lower than the reported 49-92% for preterm neonates, and higher than the reported 21% for children> 1 year of age and 30% for adults. As midazolam oral bioavailability varied widely, systemic exposure of other CYP3A-substrate drugs after oral dosing in this population may also be unpredictable, with risk of therapy failure or toxicity.

Proof of Concept: First Pediatric [14 C]microtracer Study to Create Metabolite Profiles of Midazolam

Growth and development affect drug-metabolizing enzyme activity thus could alter the metabolic profile of a drug. Traditional studies to create metabolite profiles and study the routes of excretion are unethical in children due to the high radioactive burden. To overcome this challenge, we aimed to show the feasibility of an absorption, distribution, metabolism, and excretion (ADME) study using a [14 C]midazolam microtracer as proof of concept in children. Twelve stable, critically ill children received an oral [14 C]midazolam microtracer (20 ng/kg; 60 Bq/kg) while receiving intravenous therapeutic midazolam. Blood was sampled up to 24 hours after dosing. A time-averaged plasma pool per patient was prepared reflecting the mean area under the curve plasma level, and subsequently one pool for each age group (0-1 month, 1-6 months, 0.5-2 years, and 2-6 years). For each pool [14 C]levels were quantified by accelerator mass spectrometry, and metabolites identified by high resolution mass spectrometry. Urine and feces (n = 4) were collected up to 72 hours. The approach resulted in sufficient sensitivity to quantify individual metabolites in chromatograms. [14 C]1-OH-midazolam-glucuronide was most abundant in all but one age group, followed by unchanged [14 C]midazolam and [14 C]1-OH-midazolam. The small proportion of unspecified metabolites most probably includes [14 C]midazolam-glucuronide and [14 C]4-OH-midazolam. Excretion was mainly in urine; the total recovery in urine and feces was 77-94%. This first pediatric pilot study makes clear that using a [14 C]midazolam microtracer is feasible and safe to generate metabolite profiles and study recovery in children. This approach is promising for first-in-child studies to delineate age-related variation in drug metabolite profiles.

Mini-Review: Comprehensive Drug Disposition Knowledge Generated in the Modern Human Radiolabeled ADME Study

The human radiolabeled absorption, distribution, metabolism, and excretion (ADME) study offers a quantitative and comprehensive overall picture of the disposition of a drug, including excretion pattern and metabolite profiles in circulation and excreta. The data gathered from the ADME study are highly informative for developing a cohesive strategy for clinical pharmacology studies. Elements of standard ADME study designs are described. An exciting new development in human ADME studies is the application of accelerator mass spectrometry (AMS) as the detection technique for carbon-14, in replacement of radioactivity measurements. This technology permits administration of 100-fold to 1,000-fold lower amounts of carbon-14, and thus opens the door to the application of new study designs. A new ADME study design, termed the AMS-Enabled Human ADME study, is described. In this design, both oral and intravenous administration are assessed in a single clinical study with a two-period crossover. In addition to all of the standard ADME study end points (e.g., mass balance and quantitative metabolite profiles), the AMS-Enabled ADME study can provide the fundamental pharmacokinetic parameters of clearance, volume of distribution, absolute oral bioavailability, and even estimates of the fraction of the dose absorbed. Thus, we have entered a new era of human ADME study design that can yield vastly more informative and complete data sets enabling a superior understanding of overall drug disposition.

Enteral Acetaminophen Bioavailability in Pediatric Intensive Care Patients Determined With an Oral Microtracer and Pharmacokinetic Modeling to Optimize Dosing

OBJECTIVES

Decreasing morbidity and mortality by rationalizing drug treatment in the critically ill is of paramount importance but challenging as the underlying clinical condition may lead to large variation in drug disposition and response. New microtracer methodology is now available to gain knowledge on drug disposition in the intensive care. On the basis of studies in healthy adults, physicians tend to assume that oral doses of acetaminophen will be completely absorbed and therefore prescribe the same dose per kilogram for oral and IV administration. As the oral bioavailability of acetaminophen in critically ill children is unknown, we designed a microtracer study to shed a light on this issue.

DESIGN

An innovative microtracer study design with population pharmacokinetics.

SETTING

A tertiary referral PICU.

PATIENTS

Stable critically ill children, 0-6 years old, and already receiving IV acetaminophen.

INTERVENTIONS

Concomitant administration of an oral C radiolabeled acetaminophen microtracer (3 ng/kg) with IV acetaminophen treatment (15 mg/kg every 6 hr).

MEASUREMENTS

Blood was drawn from an indwelling arterial or central venous catheter up to 24 hours after C acetaminophen microtracer administration. Acetaminophen concentrations were measured by liquid chromatography-mass spectrometry and C concentrations by accelerated mass spectrometry.

MAIN RESULTS

In 47 patients (median age of 6.1 mo; Q1-Q3, 1.8-20 mo) the mean enteral bioavailability was 72% (range, 11-91%). With a standard dose (15 mg/kg 4 times daily), therapeutic steady-state concentrations were 2.5 times more likely to be reached with IV than with oral administration.

CONCLUSIONS

Microtracer studies present a new opportunity to gain knowledge on drug disposition in the intensive care. Using this modality in children in the pediatric intensive care, we showed that enteral administration of acetaminophen results in less predictable exposure and higher likelihood of subtherapeutic blood concentration than does IV administration. IV dosing may be preferable to ensure adequate pain relief.

Human ADME for YH12852 using wavelength scanning cavity ring-down spectroscopy (WS-CRDS) after a low radioactivity dose

Aim: Human ¹⁴C radiotracer studies provide information-rich data sets that enable informed decision making in clinical drug development. These studies are supported by liquid scintillation counting after conventional-sized ¹⁴C doses (50-200 μCi) or complex accelerator mass spectrometry (AMS) after microtracer-sized doses (∼0.1-1 μCi). Mid-infrared laser-based 'cavity ring-down spectroscopy' (CRDS) is an emerging platform for the sensitive quantitation of ¹⁴C tracers. Results & methodology: We compared the total ¹⁴C concentrations in plasma and urine samples from a microtracer study using both CRDS and AMS technology. The data were evaluated using statistical and pharmacokinetic modeling. Conclusion: The CRDS method closely reproduced the AMS method for total ¹⁴C concentrations. With optimization of the automated sample interface and further testing, it promises to be an accessible, robust system for pivotal microtracer investigations.

Are Biotransformation Studies of Therapeutic Proteins Needed? Scientific Considerations and Technical Challenges

For therapeutic proteins, the currently established standard development path generally does not foresee biotransformation studies by default because it is well known that the clearance of therapeutic proteins proceeds via degradation to small peptides and individual amino acids. In contrast to small molecules, there is no general need to identify enzymes involved in biotransformation because this information is not relevant for drug-drug interaction assessment and for understanding the clearance of a therapeutic protein. Nevertheless, there are good reasons to embark on biotransformation studies, especially for complex therapeutic proteins. Typical triggers are unexpected rapid clearance, species differences in clearance not following the typical allometric relationship, a mismatch in the pharmacokinetics/pharmacodynamics (PK/PD) relationship, and the need to understand observed differences between the results of multiple bioanalytical methods (e.g., total vs. target-binding competent antibody concentrations). Early on during compound optimization, knowledge on protein biotransformation may help to design more stable drug candidates with favorable in vivo PK properties. Understanding the biotransformation of a therapeutic protein may also support designing and understanding the bioanalytical assay and ultimately the PK/PD assessment. Especially in cases where biotransformation products are pharmacologically active, quantification and assessment of their contribution to the overall pharmacological effect can be important for establishing a PK/PD relationship and extrapolation to humans. With the increasing number of complex therapeutic protein formats, the need for understanding the biotransformation of therapeutic proteins becomes more urgent. This article provides an overview on biotransformation processes, proteases involved, strategic considerations, regulatory guidelines, literature examples for in vitro and in vivo biotransformation, and technical approaches to study protein biotransformation. SIGNIFICANCE STATEMENT: Understanding the biotransformation of complex therapeutic proteins can be crucial for establishing a pharmacokinetic/pharmacodynamic relationship. This article will highlight scientific, strategic, regulatory, and technological features of protein biotransformation.

Dose-linearity of the pharmacokinetics of an intravenous [14 C]midazolam microdose in children

Aims

Drug disposition in children may vary from adults due to age‐related variation in drug metabolism. Microdose studies present an innovation to study pharmacokinetics (PK) in paediatrics; however, they should be used only when the PK is dose linear. We aimed to assess dose linearity of a [¹⁴C]midazolam microdose, by comparing the PK of an intravenous (IV) microtracer (a microdose given simultaneously with a therapeutic midazolam dose), with the PK of a single isolated microdose.

Methods

Preterm to 2‐year‐old infants admitted to the intensive care unit received [¹⁴C]midazolam IV as a microtracer or microdose, followed by dense blood sampling up to 36 hours. Plasma concentrations of [¹⁴C]midazolam and [¹⁴C]1‐hydroxy‐midazolam were determined by accelerator mass spectrometry. Noncompartmental PK analysis was performed and a population PK model was developed.

Results

Of 15 infants (median gestational age 39.4 [range 23.9–41.4] weeks, postnatal age 11.4 [0.6–49.1] weeks), 6 received a microtracer and 9 a microdose of [¹⁴C]midazolam (111 Bq kg⁻¹; 37.6 ng kg⁻¹). In a 2‐compartment PK model, bodyweight was the most significant covariate for volume of distribution. There was no statistically significant difference in any PK parameter between the microdose and microtracer, nor in the area under curve ratio [¹⁴C]1‐OH‐midazolam/[¹⁴C]midazolam, showing the PK of midazolam to be linear within the range of the therapeutic and microdoses.

Conclusion

Our data support the dose linearity of the PK of an IV [¹⁴C]midazolam microdose in children. Hence, a [¹⁴C]midazolam microdosing approach may be used as an alternative to a therapeutic dose of midazolam to study developmental changes in hepatic CYP3A activity in young children.

Radiocarbon Tracers in Toxicology and Medicine: Recent Advances in Technology and Science

This review summarizes recent developments in radiocarbon tracer technology and applications. Technologies covered include accelerator mass spectrometry (AMS), including conversion of samples to graphite, and rapid combustion to carbon dioxide to enable direct liquid sample analysis, coupling to HPLC for real-time AMS analysis, and combined molecular mass spectrometry and AMS for analyte identification and quantitation. Laser-based alternatives, such as cavity ring down spectrometry, are emerging to enable lower cost, higher throughput measurements of biological samples. Applications covered include radiocarbon dating, use of environmental atomic bomb pulse radiocarbon content for cell and protein age determination and turnover studies, and carbon source identification. Low dose toxicology applications reviewed include studies of naphthalene-DNA adduct formation, benzo[a]pyrene pharmacokinetics in humans, and triclocarban exposure and risk assessment. Cancer-related studies covered include the use of radiocarbon-labeled cells for better defining mechanisms of metastasis and the use of drug-DNA adducts as predictive biomarkers of response to chemotherapy.

Nanotracing and cavity-ring down spectroscopy: A new ultrasensitive approach in large molecule drug disposition studies

New therapeutic biological entities such as bispecific antibodies targeting tissue or specific cell populations form an increasingly important part of the drug development portfolio. However, these biopharmaceutical agents bear the risk of extensive target-mediated drug disposition or atypical pharmacokinetic properties as compared to canonical antibodies. Pharmacokinetics and bio-distribution studies become therefore more and more important during lead optimization. Biologics present, however, greater analytical challenges than small molecule drugs due to the mass and selectivity limitation of mass spectrometry and ligand-binding assay, respectively. Radiocarbon (¹⁴C) and its detection methods, such as the emerging ¹⁴C cavity ring down spectroscopy (CRDS), thus can play an important role in the large molecule quantitation where a ¹⁴C-tag is covalently bound through a stable linker. CRDS has the advantage of a simplified sample preparation and introduction system as compared to accelerator mass spectrometry (AMS) and can be accommodated within an ordinary research laboratory. In this study, we report on the labeling of an anti-IL17 IgG1 model antibody with ¹⁴C propionate tag and its detection by CRDS using it as nanotracer (2.1 nCi or 77.7 Bq blended with the therapeutic dose) in a pharmacokinetics study in a preclinical species. We compare these data to data generated by AMS in parallel processed samples. The derived concentration time profiles for anti-IL17 by CRDS were in concordance with the ones derived by AMS and γ-counting of an ¹²⁵I-labeled anti-IL17 radiotracer and were well described by a 2-compartment population pharmacokinetic model. In addition, antibody tissue distribution coefficients for anti-IL17 were determined by CRDS, which proved to be a direct and sensitive measurement of the extravascular tissue concentration of the antibody when tissue perfusion was applied. Thus, this proof-of-concept study demonstrates that trace ¹⁴C-radiolabels and CRDS are an ultrasensitive approach in (pre)clinical pharmacokinetics and bio-distribution studies of new therapeutic entities.

Successful Use of [14C]Paracetamol Microdosing to Elucidate Developmental Changes in Drug Metabolism

Background

We previously showed the practical and ethical feasibility of using [¹⁴C]-microdosing for pharmacokinetic studies in children. We now aimed to show that this approach can be used to elucidate developmental changes in drug metabolism, more specifically, glucuronidation and sulfation, using [¹⁴C]paracetamol (AAP).

Methods

Infants admitted to the intensive care unit received a single oral [¹⁴C]AAP microdose while receiving intravenous therapeutic AAP every 6 h. [¹⁴C]AAP pharmacokinetic parameters were estimated. [¹⁴C]AAP and metabolites were measured with accelerator mass spectrometry. The plasma area under the concentration-time curve from time zero to infinity and urinary recovery ratios were related to age as surrogate markers of metabolism.

Results

Fifty children [median age 6 months (range 3 days–6.9 years)] received a microdose (3.3 [2.0–3.5] ng/kg; 64 [41–71] Bq/kg). Plasma [¹⁴C]AAP apparent total clearance was 0.4 (0.1–2.6) L/h/kg, apparent volume of distribution was 1.7 (0.9–8.2) L/kg, and the half-life was 2.8 (1–7) h. With increasing age, plasma and urinary AAP-glu/AAP and AAP-glu/AAP-sul ratios significantly increased by four fold, while the AAP-sul/AAP ratio significantly decreased.

Conclusion

Using [¹⁴C]labeled microdosing, the effect of age on orally administered AAP metabolism was successfully elucidated in both plasma and urine. With minimal burden and risk, microdosing is attractive to study developmental changes in drug disposition in children.

A Decade in the MIST: Learnings from Investigations of Drug Metabolites in Drug Development under the "Metabolites in Safety Testing" Regulatory Guidance

Since the introduction of metabolites in safety testing (MIST) guidance by the Food and Drug Administration in 2008, major changes have occurred in the experimental methods for the identification and quantification of metabolites, ways to evaluate coverage of metabolites, and the timing of critical clinical and nonclinical studies to generate this information. In this cross-industry review, we discuss how the increased focus on human drug metabolites and their potential contribution to safety and drug-drug interactions has influenced the approaches taken by industry for the identification and quantitation of human drug metabolites. Before the MIST guidance was issued, the method of choice for generating comprehensive metabolite profile was radio chromatography. The MIST guidance increased the focus on human drug metabolites and their potential contribution to safety and drug-drug interactions and led to changes in the practices of drug metabolism scientists. In addition, the guidance suggested that human metabolism studies should also be accelerated, which has led to more frequent determination of human metabolite profiles from multiple ascending-dose clinical studies. Generating a comprehensive and quantitative profile of human metabolites has become a more urgent task. Together with technological advances, these events have led to a general shift of focus toward earlier human metabolism studies using high-resolution mass spectrometry and to a reduction in animal radiolabel absorption/distribution/metabolism/excretion studies. The changes induced by the MIST guidance are highlighted by six case studies included herein, reflecting different stages of implementation of the MIST guidance within the pharmaceutical industry.

Evaluation of cAMS for 14C microtracer ADME studies: opportunities to change the current drug development paradigm

Aim: Although regulatory guidances require human metabolism information of drug candidates early in the development process, the human mass balance study (or hADME study), is performed relatively late. hADME studies typically involve the administration of a 14C-radiolabelled drug where biological samples are measured by conventional scintillation counting analysis. Another approach is the administration of therapeutic doses containing a 14C-microtracer followed by accelerator mass spectrometry (AMS) analysis, enabling hADME studies completion much earlier. Consequently, there is an opportunity to change the current drug development paradigm. Materials & methods: To evaluate the applicability of the MICADAS–cAMS method, we successfully performed: the validation of MICADAS–cAMS for radioactivity quantification in biomatrices and, a rat ADME study, where the conventional methodology was assessed against a microtracer MICADAS–cAMS approach. Results & discussion: Combustion AMS (cAMS) technology is applicable to microtracer studies. A favorable opinion from EMA to complete the hADME in a Phase I setting was received, opening the possibilities to change drug development.

Use of Accelerator Mass Spectrometry in Human Health and Molecular Toxicology

Accelerator mass spectrometry (AMS) has been adopted as a powerful bioanalytical method for human studies in the areas of pharmacology and toxicology. The exquisite sensitivity (10-18 mol) of AMS has facilitated studies of toxins and drugs at environmentally and physiologically relevant concentrations in humans. Such studies include risk assessment of environmental toxicants, drug candidate selection, absolute bioavailability determination, and more recently, assessment of drug-target binding as a biomarker of response to chemotherapy. Combining AMS with complementary capabilities such as high performance liquid chromatography (HPLC) can maximize data within a single experiment and provide additional insight when assessing drugs and toxins, such as metabolic profiling. Recent advances in the AMS technology at Lawrence Livermore National Laboratory have allowed for direct coupling of AMS with complementary capabilities such as HPLC via a liquid sample moving wire interface, offering greater sensitivity compared to that of graphite-based analysis, therefore enabling the use of lower 14C and chemical doses, which are imperative for clinical testing. The aim of this review is to highlight the recent efforts in human studies using AMS, including technological advancements and discussion of the continued promise of AMS for innovative clinical based research.

The impact of early human data on clinical development: there is time to win

Modern accelerator mass spectrometry (AMS) methods enable the routine application of this technology in drug development. By the administration of a (14)C-labelled microdose or microtrace, pharmacokinetic (PK) data, such as mass balance, metabolite profiling, and absolute bioavailability (AB) data, can be generated easier, faster, and at lower costs. Here, we emphasize the advances and impact of this technology for pharmaceutical companies. The availability of accurate intravenous (iv) PK and human absorption, distribution, metabolism, and excretion (ADME) information, even before or during Phase I trials, can improve the clinical development plan. Moreover, applying the microtrace approach during early clinical development might impact the number of clinical pharmacology and preclinical safety pharmacology studies required, and shorten the overall drug discovery program.

Opportunities in low-level radiocarbon microtracing: applications and new technology

¹⁴C-radiolabeled (radiocarbon) drug studies are central to defining the disposition of therapeutics in clinical development. Concerns over radiation, however, have dissuaded investigators from conducting these studies as often as their utility may merit. Accelerator mass spectrometry (AMS), originally designed for carbon dating and geochronology, has changed the outlook for in-human radiolabeled testing. The high sensitivity of AMS affords human clinical testing with vastly reduced radiative (microtracing) and chemical exposures (microdosing). Early iterations of AMS were unsuitable for routine biomedical use due to the instruments' large size and associated per sample costs. The situation is changing with advances in the core and peripheral instrumentation. We review the important milestones in applied AMS research and recent advances in the core technology platform. We also look ahead to an entirely new class of ¹⁴C detection systems that use lasers to measure carbon dioxide in small gas cells.

Pediatric microdose study of [(14)C]paracetamol to study drug metabolism using accelerated mass spectrometry: proof of concept

BACKGROUND

Pediatric drug development is hampered by practical, ethical, and scientific challenges. Microdosing is a promising new method to obtain pharmacokinetic data in children with minimal burden and minimal risk. The use of a labeled oral microdose offers the added benefit to study intestinal and hepatic drug disposition in children already receiving an intravenous therapeutic drug dose for clinical reasons.

OBJECTIVE

The objective of this study was to present pilot data of an oral [(14)C]paracetamol [acetaminophen (AAP)] microdosing study as proof of concept to study developmental pharmacokinetics in children.

METHODS

In an open-label microdose pharmacokinetic pilot study, infants (0-6 years of age) received a single oral [(14)C]AAP microdose (3.3 ng/kg, 60 Bq/kg) in addition to intravenous therapeutic doses of AAP (15 mg/kg intravenous every 6 h). Blood samples were taken from an indwelling catheter. AAP blood concentrations were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and [(14)C]AAP and metabolites ([(14)C]AAP-Glu and [(14)C]AAP-4Sul) were measured by accelerator mass spectrometry.

RESULTS

Ten infants (aged 0.1-83.1 months) were included; one was excluded as he vomited shortly after administration. In nine patients, [(14)C]AAP and metabolites in blood samples were detectable at expected concentrations: median (range) maximum concentration (C max) [(14)C]AAP 1.68 (0.75-4.76) ng/L, [(14)C]AAP-Glu 0.88 (0.34-1.55) ng/L, and [(14)C]AAP-4Sul 0.81 (0.29-2.10) ng/L. Dose-normalized oral [(14)C]AAP C max approached median intravenous average concentrations (C av): 8.41 mg/L (3.75-23.78 mg/L) and 8.87 mg/L (3.45-12.9 mg/L), respectively.

CONCLUSIONS

We demonstrate the feasibility of using a [(14)C]labeled microdose to study AAP pharmacokinetics, including metabolite disposition, in young children.

Observational infant exploratory [(14)C]-paracetamol pharmacokinetic microdose/therapeutic dose study with accelerator mass spectrometry bioanalysis

AIMS

The aims of the study were to compare [(14)C]-paracetamol ([(14)C]-PARA) paediatric pharmacokinetics (PK) after administration mixed in a therapeutic dose or an isolated microdose and to develop further and validate accelerator mass spectrometry (AMS) bioanalysis in the 0-2 year old age group.

METHODS

[(14)C]-PARA concentrations in 10-15 µl plasma samples were measured after enteral or i.v. administration of a single [(14)C]-PARA microdose or mixed in with therapeutic dose in infants receiving PARA as part of their therapeutic regimen.

RESULTS

Thirty-four infants were included in the PARA PK analysis for this study: oral microdose (n = 4), i.v. microdose (n = 6), oral therapeutic (n = 6) and i.v. therapeutic (n = 18). The respective mean clearance (CL) values (SDs in parentheses) for these dosed groups were 1.46 (1.00) l h(-1), 1.76 (1.07) l h(-1), 2.93 (2.08) l h(-1) and 2.72 (3.10) l h(-1), t(1/2) values 2.65 h, 2.55 h, 8.36 h and 7.16 h and dose normalized AUC(0-t) (mg l(-1) h) values were 0.90 (0.43), 0.84 (0.57), 0.7 (0.79) and 0.54 (0.26).

CONCLUSIONS

All necessary ethical, scientific, clinical and regulatory procedures were put in place to conduct PK studies using enteral and systemic microdosing in two European centres. The pharmacokinetics of a therapeutic dose (mg kg(-1)) and a microdose (ng kg(-1)) in babies between 35 to 127 weeks post-menstrual age. [(14)C]-PARA pharmacokinetic parameters were within a two-fold range after a therapeutic dose or a microdose. Exploratory studies using doses significantly less than therapeutic doses may offer ethical and safety advantages with increased bionalytical sensitivity in selected exploratory paediatric pharmacokinetic studies.

Microdosing of a Carbon-14 Labeled Protein in Healthy Volunteers Accurately Predicts Its Pharmacokinetics at Therapeutic Dosages

Preclinical development of new biological entities (NBEs), such as human protein therapeutics, requires considerable expenditure of time and costs. Poor prediction of pharmacokinetics in humans further reduces net efficiency. In this study, we show for the first time that pharmacokinetic data of NBEs in humans can be successfully obtained early in the drug development process by the use of microdosing in a small group of healthy subjects combined with ultrasensitive accelerator mass spectrometry (AMS). After only minimal preclinical testing, we performed a first-in-human phase 0/phase 1 trial with a human recombinant therapeutic protein (RESCuing Alkaline Phosphatase, human recombinant placental alkaline phosphatase [hRESCAP]) to assess its safety and kinetics. Pharmacokinetic analysis showed dose linearity from microdose (53 μg) [(14) C]-hRESCAP to therapeutic doses (up to 5.3 mg) of the protein in healthy volunteers. This study demonstrates the value of a microdosing approach in a very small cohort for accelerating the clinical development of NBEs.