Creation of Novel Sensitive Probe Substrate and Moderate Inhibitor Models for a Comprehensive Prediction of CYP2C8 Interactions for Tucatinib.

A physiologically-based pharmacokinetic (PBPK) model was developed to simulate plasma concentrations of tucatinib (TUKYSA®) after single-dose or multiple-dose administration of 300 mg b.i.d. orally. This PBPK model was subsequently applied to support evaluation of drug-drug interaction (DDI) risk as a perpetrator resulting from tucatinib inhibition of CYP3A4, CYP2C8, CYP2C9, P-gp, or MATE1/2-K. The PBPK model was also applied to support evaluation of DDI risk as a victim resulting from co-administration with CYP3A4 or CYP2C8 inhibitors, or a CYP3A4 inducer. After refinement with clinical DDI data, the final PBPK model was able to recover the clinically observed single and multiple-dose plasma concentrations for tucatinib when tucatinib was administered as a single agent in healthy subjects. In addition, the final model was able to recover clinically observed plasma concentrations of tucatinib when administered in combination with itraconazole, rifampin, or gemfibrozil as well as clinically observed plasma concentrations of probe substrates of CYP3A4, CYP2C8, CYP2C9, P-gp, or MATE1/2-K. The PBPK model was then applied to prospectively predict the potential perpetrator or victim DDIs with other substrates, inducers, or inhibitors. To simulate a potential interaction with a moderate CYP2C8 inhibitor, two novel PBPK models representing a moderate CYP2C8 inhibitor and a sensitive CYP2C8 substrate were developed based on the existing PBPK models for gemfibrozil and rosiglitazone, respectively. The simulated population geometric mean area under the curve ratio of tucatinib with a moderate CYP2C8 inhibitor ranged from 1.98- to 3.08-fold, and based on these results, no dose modifications were proposed for moderate CYP2C8 inhibitors for the tucatinib label.

Effect of Tucatinib on Cardiac Repolarization in Healthy Volunteers.

BACKGROUND AND OBJECTIVE: Tucatinib is a selective tyrosine kinase inhibitor of the human epidermal growth factor receptor 2 (HER2) approved to treat metastatic HER2-positive breast and colorectal cancers. The International Council for Harmonisation of Technical Requirements for Human Use (ICH) E14 guideline mandates that new drugs are assessed for potential effects on cardiac repolarization through electrocardiogram (ECG) evaluation in a QT/corrected QT (TQT) study. METHODS: We evaluated the effect of tucatinib on cardiac repolarization in healthy volunteers in a phase I, randomized, partially double-blind, placebo-and positive-controlled three-period crossover study. The primary endpoint was the placebo-corrected change from baseline in QT interval values, corrected for heart rate using Fridericia's method (ΔΔQTcF). RESULTS: After achieving steady-state tucatinib exposures with 300 mg twice daily, the observed ΔΔQTcF ranged from -2.9 msec at 2 hours post-dose to 0 msec at 4 hours post-dose. The upper bound of the 90% confidence interval (CI) was below 5 ms at all post-dose timepoints. Assay sensitivity was confirmed as the lower bound of the 90% CI and was >5 ms following moxifloxacin dosing. As the mean ΔΔQTcF of tucatinib was predicted to be -  1.80 ms (90% CI -  3.90, 0.30) at clinically relevant tucatinib concentrations (511 ng/mL), an effect of tucatinib on QTcF exceeding 10 ms was excluded within observed ranges of tucatinib (up to ~1000 ng/mL). Tucatinib had no clinically relevant effect on heart rate or cardiac conduction. The safety profile of tucatinib was manageable after multiple doses. CONCLUSION: Tucatinib had no clinically relevant effects on studied ECG parameters. This study constitutes a clearly negative TQT study per ICH E14 guidance. CLINICAL TRIAL REGISTRATION: This trial (NCT03777761) was registered on 17 December 2018.

The Pharmacokinetics and Safety of Tucatinib in Volunteers with Hepatic Impairment.

BACKGROUND AND OBJECTIVE: Tucatinib, a highly selective tyrosine kinase inhibitor of the human epidermal growth factor receptor 2 (HER2) approved for HER2-positive metastatic breast cancer, is cleared by hepatic metabolism and subsequent biliary excretion. Liver disease can alter drug disposition and pharmacokinetics (PK). The objective of this study is to characterize PK and safety of tucatinib in volunteers with hepatic impairment. METHODS: This Phase 1 study compared the PK and safety of a single 300-mg oral dose of tucatinib in volunteers with mild, moderate, and severe hepatic impairment (Child-Pugh A/B/C) to healthy volunteers matched for sex, age, and body mass index. Pharmacokinetic parameters were determined for tucatinib and its predominant metabolite ONT-993. RESULTS: Compared with healthy volunteers, tucatinib exposure was similar in volunteers with mild impairment and increased in those with moderate or severe impairment without reaching statistical significance. Respective fold increases in geometric mean ratios for AUC0-t and AUC0-∞ were 1.13 and 1.15 in moderate impairment, and 1.43 and 1.61 in severe impairment compared with healthy volunteers. Three treatment-emergent adverse events (nausea, dermatitis, and increased transaminases) were reported in three volunteers and showed no obvious association with hepatic impairment status. CONCLUSION: The 1.61-fold geometric mean ratio AUC0-∞ increase in volunteers with severe hepatic impairment supports the recommendation in the tucatinib prescribing information to reduce the dose from 300 mg twice daily to 200 mg twice daily in patients with severe impairment; no dose adjustment is recommended for patients with mild or moderate hepatic impairment. This trial (NCT03722823) was registered on October 29, 2018.

Evaluation of Safety and Clinically Relevant Drug-Drug Interactions with Tucatinib in Healthy Volunteers.

BACKGROUND AND OBJECTIVE: Tucatinib is approved for treatment of human epidermal growth factor receptor 2-positive metastatic breast cancer. Understanding potential drug-drug interactions (DDIs) informs proper dosing when co-administering tucatinib with other therapies. The aim of this study was to evaluate DDIs between tucatinib and metabolizing enzymes and transporters in healthy volunteers. METHODS: Parts A-C assessed the impact of itraconazole (cytochrome P450 [CYP] 3A4 inhibitor), rifampin (CYP3A4/CYP2C8 inducer), or gemfibrozil (CYP2C8 inhibitor) on the pharmacokinetics of a single 300 mg dose of tucatinib administered orally and its primary metabolite, ONT-993. Parts D and E assessed the effect of steady-state tucatinib on the pharmacokinetics of repaglinide (CYP2C8 substrate), tolbutamide (CYP2C9 substrate), midazolam (CYP3A4 substrate), and digoxin (P-glycoprotein substrate). RESULTS: Tucatinib area under the concentration-time curve from time 0 extrapolated to infinity (AUC0-inf) increased by ~ 1.3- and 3.0-fold with itraconazole and gemfibrozil, respectively, and decreased by 48% with rifampin, indicating that tucatinib is metabolized primarily by CYP2C8, and to a lesser extent via CYP3A. Tucatinib was a strong inhibitor of CYP3A (midazolam AUC0-inf increased 5.7-fold), a weak inhibitor of CYP2C8 and P-glycoprotein, and had no impact on CYP2C9-mediated metabolism in humans. Tucatinib was well tolerated, alone and with co-administered drugs. CONCLUSION: The potential DDIs identified here may be mitigated by avoiding concomitant use of tucatinib with strong CYP3A inducers, moderate CYP2C8 inducers, CYP3A substrates with a narrow therapeutic window (modifying substrate dose where concomitant use is unavoidable), and strong CYP2C8 inhibitors (decreasing tucatinib dose where concomitant use is unavoidable), or by reducing the dose of P-glycoprotein substrates with a narrow therapeutic window. TRIAL REGISTRATION: This trial (NCT03723395) was registered on October 29, 2018.

Elimination of tucatinib, a small molecule kinase inhibitor of HER2, is primarily governed by CYP2C8 enantioselective oxidation of gem-dimethyl.

PURPOSE: Tucatinib, a small molecule for the treatment of metastatic HER2-positive breast cancer, was extensively metabolized in humans to multiple oxidative metabolites. To fully understand the elimination and biotransformation pathways of tucatinib, we investigated the in vitro and in vivo metabolism of tucatinib, and also conducted a Phase I trial using [14C]tucatinib. METHODS: To identify the responsible enzymes for tucatinib clearance, we investigated the in vitro metabolism of tucatinib including enzyme phenotyping, which facilitated the discovery of several metabolites in human and monkey plasma and excreta, in particular M1 (ONT-993, an aliphatic hydroxylated metabolite). Stereoselective formation of M1 was further investigated in vitro, in vivo, and in silico. RESULTS: In humans, approximately 86% of the total radiolabeled dose was recovered in feces and 4% in urine; in plasma, approximately 76% of radioactivity circulated as parent drug, with 19% attributed to multiple metabolites. The primary isoforms responsible for the elimination of tucatinib were CYP2C8 and CYP3A4/5. CYP2C8 was shown to possess sole catalytic activity for the formation of M1, whereas CYP3A4/5 and aldehyde oxidase catalyzed the formation of the remaining metabolites. Subsequent investigation revealed that M1 was formed in a stereoselective manner. Examination of the enantiomeric ratio of M1 stereoisomers observed in humans relative to cynomolgus monkeys revealed comparable results, suggesting that the enantiomers that comprise M1 were not considered to be unique or disproportionately high in human. CONCLUSION: CYP2C8 and CYP3A4/5 are the primary drug-metabolizing enzymes involved in the in vitro metabolism of tucatinib, which provided the basis to describe human disposition of tucatinib and formation of the observed metabolites.

Risk-Based Pharmacokinetic and Drug-Drug Interaction Characterization of Antibody-Drug Conjugates in Oncology Clinical Development: An International Consortium for Innovation and Quality in Pharmaceutical Development Perspective.

Antibody-drug conjugates (ADCs) represent a rapidly evolving area of drug development and hold significant promise. To date, nine ADCs have been approved by the US Food and Drug Administration (FDA). These conjugates combine the target specificity of monoclonal antibodies with the anticancer activity of small-molecule therapeutics (also referred to as payload). Due to the complex structure, three analytes, namely ADC conjugate, total antibody, and unconjugated payload, are typically quantified during drug development; however, the benefits of measuring all three analytes at later stages of clinical development are not clear. The cytotoxic payloads, upon release from the ADC, are considered to behave like small molecules. Given the relatively high potency and low systemic exposure of cytotoxic payloads, drug-drug interaction (DDI) considerations for ADCs might be different from traditional small molecule therapeutics. The International Consortium for Innovation and Quality in Pharmaceutical Development (IQ Consortium) convened an ADC working group to create an IQ ADC database that includes 26 ADCs with six unique payloads. The analysis of the ADC data in the IQ database, as well as nine approved ADCs, supports the strategy of pharmacokinetic characterization of all three analytes in early-phase development and progressively minimizing the number of analytes to be measured in the late-phase studies. The systemic concentrations of unconjugated payload are usually too low to serve as a DDI perpetrator; however, the potential for unconjugated payloads as a victim still exists. A data-driven and risk-based decision tree was developed to guide the assessment of a circulating payload as a victim of DDI.

Tucatinib Inhibits Renal Transporters OCT2 and MATE Without Impacting Renal Function in Healthy Subjects.

Tucatinib is a potent tyrosine kinase inhibitor selective for human epidermal growth factor receptor 2 (HER2) approved by the US Food and Drug Administration for the treatment of HER2-positive metastatic breast cancer and in development for other HER2-positive solid tumors. Modest, reversible serum creatinine (SCr) elevations have been observed in tucatinib clinical trials. SCr is conveyed by the renal drug transporters organic cation transporter 2 (OCT2) and multidrug and toxin extrusion protein 1 (MATE1) and 2-K (MATE2-K) and can increase in the presence of inhibitors of these transporters. In vitro, tucatinib inhibited OCT2-, MATE1-, and MATE2-K-mediated transport of metformin, with IC50 values of 14.7, 0.340, and 0.135 µM, respectively. Tucatinib also inhibited OCT2- and MATE1-mediated transport of creatinine, with IC50 values of 0.107 and 0.0855 µM, respectively. A phase 1 study with metformin administered orally in the absence and presence of tucatinib was conducted in 18 healthy subjects. Renal function was assessed by measuring glomerular filtration rate (GFR; based on iohexol plasma clearance) and endogenous markers (SCr, cystatin C-based estimated glomerular filtration rate [eGFR]) with and without tucatinib. Metformin exposure increased (1.4-fold) and renal clearance decreased (29.99-17.64 L/h) with tucatinib, with no effect on metformin maximum concentration. Creatinine clearance transiently decreased 23% with tucatinib. GFR and eGFR, which are unaffected by OCT2 and/or MATE1/2-K transport, were unchanged with tucatinib. These data demonstrate that tucatinib inhibits OCT2- and MATE1/2-K-mediated tubular secretion of creatinine, which may manifest as mild SCr elevations that are not indicative of renal impairment.

Population Pharmacokinetic Modeling and Exposure-Response Assessment for the Antibody-Drug Conjugate Brentuximab Vedotin in Hodgkin's Lymphoma in the Phase III ECHELON-1 Study.

The efficacy of the CD30-directed antibody-drug conjugate (ADC) brentuximab vedotin was established in combination with chemotherapy as frontline treatment for advanced classical Hodgkin's lymphoma in the randomized phase III ECHELON-1 study. Population pharmacokinetic (PK) and exposure-response models were developed to quantify sources of PK variability and relationships between exposure and safety/efficacy end points in ECHELON-1. The influence of patient-specific factors on the PK of the ADC and the microtubule-disrupting payload monomethyl auristatin E (MMAE) was investigated; none of the significant covariates had a clinically relevant impact. Exposure-response analyses evaluated relationships between time-averaged area under the curve (AUC; ADC, MMAE) and efficacy end points (ADC) or safety parameters (ADC, MMAE). Exposure-efficacy analyses supported consistent treatment benefit with brentuximab vedotin across observed exposure ranges. Exposure-safety analyses supported the recommended brentuximab vedotin starting dose (1.2 mg/kg every 2 weeks), and effective management of peripheral neuropathy and neutropenia with dose modification/reduction and febrile neutropenia with granulocyte colony-stimulating factor primary prophylaxis.

Microglial activation is inversely associated with cognition in individuals living with HIV on effective antiretroviral therapy.

OBJECTIVE: Despite viral suppression, HIV-associated cognitive impairment persists and may be partially due to persistent immune signalling by cells of the myeloid-lineage. Here, we aimed to understand the contribution of activated microglia located in vulnerable brain regions (e.g. frontal, subcortical) of HIV-infected, virally suppressed (HIV+VS) individuals in relation to cognitive and motor function. DESIGN: Twenty-one HIV+VS individuals underwent PET with [11C]DPA-713 to image the translocator protein 18 kDa (TSPO), a marker of microglial activation, and completed a comprehensive neuropsychological test battery. METHODS: Multivariable linear regressions were used to examine the contribution of [11C]DPA-713 binding to cognitive performance. RESULTS: Higher [11C]DPA-713 binding was associated with lower cognition among HIV+VS individuals. [11C]DPA-713 binding in middle frontal gyrus/frontal cortex, hippocampus/temporal cortex and occipital cortex was inversely associated with performance on a number of cognitive domains, including verbal memory, processing speed/attention/concentration, executive function, working memory and motor function. [C]DPA-713 binding in parietal cortex, cerebellum and thalamus was associated with only specific cognitive domains including visual construction and verbal memory. Binding was not associated with global cognitive performance. CONCLUSION: The findings add to the growing body of evidence that immune-mediated brain injury may contribute to domain specific, HIV-associated, cognitive vulnerabilities despite viral suppression.

Neuroimaging of translocator protein in patients with systemic lupus erythematosus: a pilot study using [11C]DPA-713 positron emission tomography.

Objective Inflammation secondary to autoantibody-mediated effects occurring in multiple organs is a hallmark of systemic lupus erythematosus (SLE). The inflammatory response to SLE-mediated damage in brain parenchyma has been postulated in both normal and cognitively impaired individuals. Our goal is to use molecular imaging to investigate the distribution within the brain of the mitochondrial translocator protein (TSPO) that is upregulated during glial cell activation, and is considered as a marker of brain injury and repair. Methods We sought to characterize TSPO distribution in the brain of SLE patients using positron emission tomography (PET) and [11C]DPA-713 (DPA), a radiopharmaceutical that targets TSPO. We imaged 11 healthy controls and 10 patients with SLE (years of diagnosis: 13.0 ± 7.7), all between the ages of 22 and 52. RESULTS: Among the nine brain regions studied, no statistically significant increases in DPA binding were observed in SLE. Instead, there was a significant decrease in TSPO distribution in the cerebellum and hippocampus of SLE patients, as compared to healthy controls. Such decreases were most significant in cognitively normal SLE subjects, but showed pseudo-normalization in those with cognitive impairment, due to higher cerebellar and hippocampal DPA binding in the cognitively impaired (versus normal) SLE brain. Conclusions Results from this pilot study suggest a link between diminished regional TSPO expression in the brain of patients with SLE, as well as possible glial cell activation within the cerebellum and hippocampus of cognitively impaired individuals with SLE. Further studies are needed to elucidate how mitochondrial dysfunction and glial cell activation may act together in SLE and SLE-mediated neurocognitive deficits.

The role of the equilibrative nucleoside transporter 1 on tissue and fetal distribution of ribavirin in the mouse.

Ribavirin is used for the treatment of hepatitis C virus (HCV) infection. The equilibrative nucleoside transporter 1 (ENT1) expressed in hepatocytes transports ribavirin into the liver, the site of efficacy of the drug. However, it is still unclear whether ENT1 plays a dominant role in the hepatic distribution of the drug in vivo. In addition, due to fetal toxicity, administration of ribavirin to pregnant women with HCV infection is contraindicated. ENT1 might play a role in the fetal distribution and therefore the fetal toxicity of ribavirin. The aim of the present study was to investigate the in vivo contribution of ENT1 to the tissue distribution of ribavirin. When compared with that in Ent1(+/+) mice, the ribavirin tissue to plasma concentration ratio (including phosphorylated metabolites) in Ent1(-/-) mice at 15 min and 6 h after intravenous [(3) H]-ribavirin (3 mg/kg) administration was consistently and significantly decreased in the liver and the pancreas. Likewise, when compared with the Ent1(+/+) mice, the fetal distribution of ribavirin at 15 min after administration was significantly reduced in Ent1(-/-) fetuses and placenta. In contrast, there was no significant difference between Ent1(+/+), Ent1(+/-) and Ent1(-/-) mice in the fetal or placental to maternal plasma ribavirin concentration ratio at 2 h after ribavirin administration. The findings in the present study suggest that ENT1 plays a pivotal role in the distribution of ribavirin into tissues including the liver and pancreas, but affects only the rate, but not the extent, of ribavirin distribution into the fetus. Copyright © 2016 John Wiley & Sons, Ltd.

Population pharmacokinetics of brodalumab in healthy adults and adults with psoriasis from single and multiple dose studies.

Brodalumab, a human monoclonal IgG2-antibody, acts as a potent antagonist at the interleukin-17 receptor A, which is important in the pathogenesis of psoriasis. To characterize the pharmacokinetics of brodalumab and assess the effects of covariates, brodalumab concentrations from Phase 1a and Phase 2 clinical studies in healthy adults and subjects with psoriasis were used to construct a population PK model. The final two-compartment model with parallel linear and non-linear elimination pathways fit the data well. The population typical values for PK parameters CL, V, and V(max) were 0.223 L/day, 4.62 L, and 5.40 mg/day with between-subject-variability of 69.2, 69.6, and 25.9%CV, respectively. Body weight (BW) was an important covariate on CL (and Q), V (and V(2)) and V(max), with estimated effect exponents of 0.598, 0.849, and 1.12, respectively. Based on simulations from the final model, for doses between 140 and 210 mg, AUC was predicted to be greater than two fold higher in subjects weighing less than 75 kg compared to reference subjects. Age and diagnosis had smaller influence on exposure and was not clinically significant. These data suggest that BW is an important covariate explaining some of the variability in population PK observed in human clinical trials with brodalumab.

Regional brain distribution of translocator protein using [(11)C]DPA-713 PET in individuals infected with HIV.

Imaging the brain distribution of translocator protein (TSPO), a putative biomarker for glial cell activation and neuroinflammation, may inform management of individuals infected with HIV by uncovering regional abnormalities related to neurocognitive deficits and enable non-invasive therapeutic monitoring. Using the second-generation TSPO-targeted radiotracer, [(11)C]DPA-713, we conducted a positron emission tomography (PET) study to compare the brains of 12 healthy human subjects to those of 23 individuals with HIV who were effectively treated with combination antiretroviral therapy (cART). Compared to PET data from age-matched healthy control subjects, [(11)C]DPA-713 PET of individuals infected with HIV demonstrated significantly higher volume-of-distribution (VT) ratios in white matter, cingulate cortex, and supramarginal gyrus, relative to overall gray matter VT, suggesting localized glial cell activation in susceptible regions. Regional TSPO abnormalities were evident within a sub-cohort of neuro-asymptomatic HIV subjects, and an increase in the VT ratio within frontal cortex was specifically linked to individuals affected with HIV-associated dementia. These findings were enabled by employing a gray matter normalization approach for PET data quantification, which improved test-retest reproducibility, intra-class correlation within the healthy control cohort, and sensitivity of uncovering abnormal regional findings.

Sex differences in translocator protein 18 kDa (TSPO) in the heart: implications for imaging myocardial inflammation.

Myocarditis is more severe in men than in women and difficult to diagnose due to a lack of imaging modalities that directly detect myocardial inflammation. Translocator protein 18 kDa (TSPO) is used extensively to image brain inflammation due to its presence in CD11b(+) brain microglia. In this study, we examined expression of TSPO and CD11b in mice with coxsackievirus B3 (CVB3) myocarditis and biopsy sections from myocarditis patients in order to determine if it could be used to image myocarditis. We found that male mice with CVB3 myocarditis upregulated more genes associated with TSPO activation than female mice. TSPO expression was increased in the heart of male mice and men with myocarditis compared with female subjects due to testosterone, where it was expressed predominantly in CD11b(+) immune cells. We show that TSPO ligands detect myocardial inflammation using microSPECT, with increased uptake of [(125)I]-IodoDPA-713 in male mice with CVB3 myocarditis compared with undiseased controls.

A semi-mechanistic model to characterize the pharmacokinetics and pharmacodynamics of brodalumab in healthy volunteers and subjects with psoriasis in a first-in-human single ascending dose study.

Pharmacokinetic-pharmacodynamic (PK-PD) modeling can provide a framework for quantitative "learning and confirming" from studies in all phases of drug development. Brodalumab is a human monoclonal antibody (IgG2 ) targeting the IL-17 receptor A that blocks signaling by cytokines thought to play a central role in the pathogenesis of psoriasis (IL-17A, IL-17F, and IL-17A/F). We used semi-mechanistic modeling of single dose, first-in-human data to characterize the exposure-response relationship between brodalumab and the Psoriasis Area and Severity Index (PASI) in a Phase 1 clinical trial. Fifty-seven healthy volunteers and 25 subjects with moderate to severe psoriasis received single intravenous or subcutaneous administration of placebo or brodalumab (7-700 mg). A two-compartment model with parallel linear and nonlinear (Michaelis-Menten) elimination pathways described brodalumab PK. The PK-PASI relationship was characterized by linking a signaling compartment with an indirect response model of psoriatic plaques, where signaling suppressed plaque formation. The concentration of half-maximal inhibition IC50 was 2.86 µg/mL (SE: 50%). The endogenous psoriatic plaque formation rate of 0.862 (SE: 40%) PASI units/day was comparable with literature precedent. Despite the small sample size and single administration data, this semi-mechanistic modeling approach provided a quantitative framework to inform design of dose-ranging Phase 2 studies of brodalumab in psoriasis.

Radiochemical synthesis and in vivo evaluation of [18F]AZ11637326: an agonist probe for the α7 nicotinic acetylcholine receptor.

INTRODUCTION: The alpha-7 nicotinic acetylcholine receptor (α7 nAChR) is key in brain communication and has been implicated in the pathophysiology of diseases of the central nervous system. A positron-emitting radioligand targeting the α7 nAChR would enable better understanding of a variety of neuropsychiatric illnesses, including schizophrenia and Alzheimer's disease, and could enhance the development of new drugs for these and other conditions. We describe our attempt to synthesize an α7 nAChR-selective radiotracer for positron emission tomography (PET). METHODS: We prepared the high-affinity (K(d) = 0.2 nM) α7 nAChR agonist, 5'-(2-[(18)F]fluorophenyl)spiro[1-azabicyclo-[2.2.2]octane]-3,2'-(3'H)furo[2,3-b]pyridine, [(18)F]AZ11637326, in two steps, a nucleophilic fluorination followed by decarbonylation. We studied [(18)F]AZ11637326 in rodents, including mice lacking α7 nAChR, and in non-human primates. RESULTS: [(18)F]AZ11637326 was synthesized in a non-decay-corrected radiochemical yield of 3% from the end of synthesis (90 min) with a radiochemical purity >90% and average specific radioactivity of 140GBq/µmol (3,781 mCi/µmol). Modest rodent brain uptake was observed (2-5% injected dose per gram of tissue, depending on specific activity), with studies comparing CD-1 and α7 nAChR null mice indicating an element of target-specific binding. Blocking studies in non-human primates did not reveal specific binding within the brain. CONCLUSION: Despite the high affinity and target selectivity of AZ11637326 for α7 nAChR in vitro and encouraging rodent studies, receptor-mediated binding could not be demonstrated in non-human primates. Further structural optimization of compounds of this class will be required for them to serve as suitable radiotracers for PET.

Molecular imaging of conscious, unrestrained mice with AwakeSPECT.

UNLABELLED: We have developed a SPECT imaging system, AwakeSPECT, to enable molecular brain imaging of untrained mice that are conscious, unanesthetized, and unrestrained. We accomplished this with head tracking and motion correction techniques. METHODS: The capability of the system for motion-corrected imaging was demonstrated with a (99m)Tc-pertechnetate phantom, (99m)Tc-methylene diphosphonate bone imaging, and measurement of the binding potential of the dopamine transporter radioligand (123)I-ioflupane in mouse brain in the awake and anesthetized (isoflurane) states. Stress induced by imaging in the awake state was assessed through measurement of plasma corticosterone levels. RESULTS: AwakeSPECT provided high-resolution bone images reminiscent of those obtained from CT. The binding potential of (123)I-ioflupane in the awake state was on the order of 50% of that obtained with the animal under anesthesia, consistent with previous studies in nonhuman primates. Levels of stress induced were on the order of those seen in other behavioral tasks and imaging studies of awake animals. CONCLUSION: These results demonstrate the feasibility of SPECT molecular brain imaging of mice in the conscious, unrestrained state and demonstrate the effects of isoflurane anesthesia on radiotracer uptake.

Biodistribution, tumor detection, and radiation dosimetry of 18F-DCFBC, a low-molecular-weight inhibitor of prostate-specific membrane antigen, in patients with metastatic prostate cancer.

UNLABELLED: Prostate-specific membrane antigen (PSMA) is a type II integral membrane protein expressed on the surface of prostate cancer (PCa) cells, particularly in androgen-independent, advanced, and metastatic disease. Previously, we demonstrated that N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-4-(18)F-fluorobenzyl-L-cysteine ((18)F-DCFBC) could image an experimental model of PSMA-positive PCa using PET. Here, we describe the initial clinical experience and radiation dosimetry of (18)F-DCFBC in men with metastatic PCa. METHODS: Five patients with radiologic evidence of metastatic PCa were studied after the intravenous administration of 370 MBq (10 mCi) of (18)F-DCFBC. Serial PET was performed until 2 h after administration. Time-activity curves were generated for selected normal tissues and metastatic foci. Radiation dose estimates were calculated using OLINDA/EXM 1.1. RESULTS: Most vascular organs demonstrated a slow decrease in radioactivity concentration over time consistent with clearance from the blood pool, with primarily urinary radiotracer excretion. Thirty-two PET-positive suspected metastatic sites were identified, with 21 concordant on both PET and conventional imaging for abnormal findings compatible with metastatic disease. Of the 11 PET-positive sites not identified on conventional imaging, most were within the bone and could be considered suggestive for the detection of early bone metastases, although further validation is needed. The highest mean absorbed dose per unit administered radioactivity (µGy/MBq) was in the bladder wall (32.4), and the resultant effective dose was 19.9 ± 1.34 µSv/MBq (mean ± SD). CONCLUSION: Although further studies are needed for validation, our findings demonstrate the potential of (18)F-DCFBC as a new positron-emitting imaging agent for the detection of metastatic PCa. This study also provides dose estimates for (18)F-DCFBC that are comparable to those of other PET radiopharmaceuticals such as (18)F-FDG.

Role of the equilibrative and concentrative nucleoside transporters in the intestinal absorption of the nucleoside drug, ribavirin, in wild-type and Ent1(-/-) mice.

Ribavirin is frontline treatment for hepatitis C virus infection. To determine the role of nucleoside transporters in the intestinal absorption of orally administered ribavirin, we perfused the intestines of Ent1(-/-) and wild-type mice, in situ, with [(3)H] ribavirin (20, 200, and 5000 µM) in the presence and absence of sodium. The decrease in luminal ribavirin concentration over 30 min was measured at 5 min intervals. Blood samples were collected approximately every 10 min. Ribavirin plus phosphorylated metabolite concentrations (hereafter referred to as ribavirin) were determined in tissue, blood, and plasma by HPLC fractionation and scintillation counting. There was no significant difference between wild-type and Ent1(-/-) mice in intestinal loss of ribavirin at any ribavirin concentration studied. Perfusions without sodium drastically reduced the intestinal loss of ribavirin in both wild-type and Ent1(-/-) mice. After 20 µM ribavirin perfusions, Ent1(-/-) intestinal tissue contained 8-fold greater ribavirin than wild-type mice (p < 0.01). Ribavirin concentrations in the wild-type intestinal tissue were 70-fold higher after 200 vs 20 µM perfusions (p < 0.001), indicating saturation of intestinal ribavirin efflux and possibly other processes as well. Ribavirin plasma concentrations were significantly higher in wild-type mice (2.7-fold) vs Ent1(-/-) mice at 30 min after the 20 µM perfusion (p < 0.01). These results suggest that, at lower intestinal concentrations of ribavirin, concentrative and equilibrative nucleoside transporters are important in the intestinal absorption of ribavirin. At higher intestinal concentrations, these transporters are saturated and other processes in the intestine (transport and/or metabolism) play an important role in the absorption of ribavirin.

Radiation dosimetry and biodistribution of the TSPO ligand 11C-DPA-713 in humans.

UNLABELLED: Whole-body PET/CT was used to characterize the radiation dosimetry of (11)C-DPA-713, a specific PET ligand for the assessment of translocator protein. METHODS: Six healthy control subjects, 3 men and 3 women, underwent whole-body dynamic PET scans after bolus injection of (11)C-DPA-713. Subjects were scanned from head to mid thigh with 7 passes performed, with a total PET acquisition of approximately 100 min. Time-activity curves were generated in organs with visible tracer uptake, and tissue residence times were calculated. Whole-body dosimetry was calculated using OLINDA 1.1 software, assuming no voiding. RESULTS: The absorbed dose is highest in the lungs, spleen, kidney, and pancreas. The lungs were determined to be the dose-limiting organ, with an average absorbed dose of 2.01 × 10(-2) mSv/MBq (7.43 × 10(-2) rem/mCi). On the basis of exposure limits outlined in the U.S. Food and Drug Administration Code of Federal Regulations (21CFR361.1), the single-dose limit for (11)C-DPA-713 radiotracer injection is 2,487.6 MBq (67.3 mCi). CONCLUSION: (11)C-DPA-713 has an uptake pattern that is consistent with the biodistribution of translocator protein and yields a dose burden that is comparable to that of other (11)C-labeled PET tracers.