PurposeDopamine D2/3 receptors are expressed throughout the human brain, but synchronous receptor quantification in subcortical and cortical brain areas remains difficult to achieve with a single radioligand. Here, we assessed the suitability of a bolus plus constant infusion (B/I) paradigm for brain-wide dopamine D2/3 receptor imaging with the agonist radioligand [11C]-(+)-PHNO.MethodsFive healthy male volunteers underwent two B/I [11C]-(+)-PHNO positron emission tomography (PET) scans combined with a mid-scan intravenous amphetamine challenge. Non-displaceable binding potentials (BPND) were calculated using the equilibrium ratio method and an algorithmically constrained cerebellum as the reference region.ResultsReproducible baseline BPND values were obtained for the majority of regions examined, with intraclass correlation coefficients ranging from 0.5 to 0.93, and test-retest variability remaining below 10%. Moreover, amphetamine-induced decreases in receptor binding were observed in both scans in cortical and a number of subcortical regions. In the globus pallidus and ventral striatum, however, equilibrium had not been reached prior to amphetamine injection, suggesting that further optimization of the protocol might be needed to quantify amphetamine effects in D3 receptor-rich regions.ConclusionsThese results indicate that, within certain limitations, synchronous cortical and subcortical measurements of D2/3 receptors at baseline and under challenge conditions can be achieved using the B/I approach and [11C]-(+)-PHNO PET.

OBJECTIVES:This study measured the penetration of ceftaroline and ceftazidime/avibactam into cerebrospinal fluid (CSF) to evaluate the potential of both drugs for treatment of central nervous system (CNS) infections. METHODS:In this prospective, single-centre pharmacokinetic (PK) study, 24 healthy volunteers equally divided into two groups received four doses of either 600 mg ceftaroline fosamil or 2000/500 mg ceftazidime/avibactam as intravenous infusions over 2 h at 8 h intervals for 4 doses. Plasma samples were obtained on both study days and CSF was sampled once per subject at either 2 h, 4 h or 8 h after the start of the last infusion via lumbar puncture. PK data were analysed using non-compartmental analysis, as well as using a population PK modelling approach. Monte Carlo simulations were performed to calculate probability of PK-pharmacodynamic (PK-PD) target attainment. RESULTS:Two-compartment models described the plasma PK data for all three compounds. The ratios between the estimated distribution clearances into and out of the CSF were 0.021, 0.083 and 0.071 for ceftaroline, ceftazidime and avibactam, respectively, indicating limited CSF penetration. Ceftaroline and ceftazidime PK-PD targets were not attained in CSF for minimum inhibitory concentrations around commonly used susceptibility breakpoints, but exposure appears sufficient to treat several pathogens commonly causing CNS infections. Avibactam concentrations were well below reported threshold concentrations that are required for activity. CONCLUSION:In healthy subjects, ceftaroline, ceftazidime and avibactam poorly distribute to CSF. Nonetheless, CSF exposure of both cephalosporins might be sufficient to cover certain, but not all, pathogens causative of CNS infections.

BACKGROUND:Because of the initially urgent need for treatments for COVID-19-associated acute respiratory distress syndrome (ARDS), the efficacy and tolerability of inhaled solnatide, a direct activator of the epithelial sodium channel (ENaC) under clinical investigation for the treatment of ARDS, were assessed in a pilot trial. METHODS:This randomized controlled double-blind clinical trial was performed at two study centers in Vienna, Austria. Adult mechanically ventilated patients with moderate to severe ARDS (Berlin Definition) caused by COVID-19 were randomized 1:1 to inhalation of solnatide (100 mg) or placebo twice daily for seven days. The primary outcome parameter was ventilator-free days (VFDs) within 28 days; survival was assessed at 28 and 60 days as a secondary outcome. RESULTS:A total of 30 out of the 40 planned patients were included, 15 randomized to solnatide and 15 to placebo. Then the trial was stopped early due to slow recruitment. The median VFDs were 0 in both groups (p =  0.653). Twenty out of 30 patients (66.7%) survived until day 28 [solnatide group: n = 11 (73.3%), placebo group: n = 9 (60%)]. One additional patient per group died until day 60. In total, 41 adverse events (AEs) and 10 serious AEs occurred in the solnatide group, and 26 AEs and 14 serious AEs in the placebo group. No AE was considered to be related to the study treatment. CONCLUSIONS:There was no difference in VFDs or survival between the solnatide and the placebo group. Solnatide appeared safe in this limited cohort of critically ill patients with COVID-19. REGISTRATION:EU clinical trials register, EudraCT number 2020-001244-26.

Excessive production of reactive oxygen species (ROS), particularly superoxide anion ( O_2^· - ), is a key mechanism in diseases such as cancer, inflammatory disorders, neurodegenerative conditions, and metabolic diseases. Evidence also suggests that microgravity-induced oxidative stress, primarily driven by elevated O_2^· - levels, may contribute to the adverse physiological effects observed in astronauts during extended space missions. Superoxide dismutase (SOD) is critical for mitigating oxidative stress, and exogenous SOD supplementation offers a potential therapeutic strategy. This randomized, double-blind, placebo-controlled Phase I study evaluated the safety, tolerability, and pharmacokinetics of recombinant human Cu/Zn-SOD (rhSOD, SOD1) following subcutaneous administration of 40 mg every 12 hours in 16 healthy volunteers. Eight subjects were enrolled each in the single-dose (SD) and multiple-dose (MD) cohorts. Study assessments, including pharmacokinetic sampling, were performed for 72 (SD) or 92 hours (MD). Injection site erythema was objectively assessed using the innovative Standardized Erythema Value (SEV*) method, derived from photographs taken with Scarletred®Vision software (SCARLETRED Holding GmbH). No serious adverse events occurred, and all treatment-related adverse events were mild. Injection site erythema was objectively assessed using the innovative standardized erythema value (SEV*) method, derived from photographs taken with Scarletred® Vision software (SCARLETRED Holding GmbH). The Visual Analog Scale scores and SEV* assessments were comparable between the rhSOD and placebo groups. Beyond safety and tolerability, pharmacokinetic analysis revealed that the volume of distribution, clearance, and half-life at presumed steady state were 129 ± 66.3 L, 5.97 ± 1.25 L/h, and 15.0 ± 6.69 h, respectively. Compared with the rapid systemic elimination after intravenous administration of SOD, subcutaneous administration resulted in a favorable plasma concentration-time profile. These findings suggest that subcutaneous rhSOD may be a promising therapeutic candidate for conditions characterized by excessive O_2^· - exposure or diminished endogenous SOD activity. Further clinical studies are warranted to assess its anti-inflammatory potential in relevant patient populations. EudraCT Number: 2022-000173-11.

An important safety consideration for topically administered drugs is the extent of systemic exposure they achieve. The aim of this study was to evaluate whether a topically administered microdose of the model drug diclofenac can predict the systemic availability, plasma, and tissue pharmacokinetics of a topical therapeutic dose. Eight healthy participants (6 men and 2 women) participated in a 4-period, crossover study. In period 1, a topical microdose (62 ± 6 μg) was administered; in period 2, a single intravenous microdose (0.95 ± 0.03 μg) was administered; in period 3, a topical therapeutic dose (120 mg) was administered; and in period 4, a single intravenous therapeutic dose (75 mg) of [14C]diclofenac was administered, with or without the addition of unlabeled diclofenac. Venous blood, urine, and microdialysis samples from subcutaneous adipose tissue beneath the dermal application site were collected for 24 h post-dosing. Total 14C-concentrations in plasma and microdialysates were quantified using accelerator mass spectrometry. The disposition of intravenously administered [14C]diclofenac was dose-linear. However, after topical administration, the fraction of total 14C absorbed (geometric mean and 95% confidence interval) was higher (P = .0019, 2-tailed, paired t test) for the microdose (0.48% and 0.34%-0.67%) compared with the therapeutic dose (0.13% and 0.07%-0.22%) (geometric mean ratio and 90% confidence interval: 3.79 and 2.41-5.98). Dose-normalized 14C-concentrations in microdialysates were low, variable, and did not differ between doses. Our study demonstrates the feasibility of quantifying 14C-concentrations in plasma and microdialysates following the topical administration of a microdose of [14C]diclofenac. The observed nonlinearity in systemic availability after topical dosing suggests that microdosing may not accurately predict the disposition of certain topical drugs at therapeutic doses. SIGNIFICANCE STATEMENT: We assessed whether a topically applied microdose of diclofenac could predict the systemic availability of a therapeutic dose. Results showed that systemic absorption was not dose-linear, indicating that microdosing may have limited use for predicting the pharmacokinetics of some topical drugs.

OBJECTIVE:The objective of this study was to assess the safety and tolerability of the intratympanic delivery of AC102, a novel pharmaceutical therapy based on a thermosensitive gel for preventing and treating a range of hearing impairments, including sudden sensorineural hearing loss. We studied this in healthy, normal-hearing volunteers to evaluate any change in hearing thresholds. STUDY DESIGN:An open-label, placebo-controlled, ascending single-dose, multicenter phase 1 clinical trial. SETTING:The study was conducted in two centers (blinded for reviewing purposes). SUBJECTS:Forty-two normal-hearing healthy volunteers younger than 40 years of age were eligible for enrollment in the study. INTERVENTION:A single intratympanic injection of a thermosensitive gel, either a placebo or containing AC102, to the middle ear. MAIN OUTCOME MEASURE:The primary objective of this study was to assess the safety and tolerability of a single intratympanic injection of ascending volume of placebo and ascending volume and concentration of AC102. The secondary objective was to determine single-dose pharmacokinetics of intratympanically injected AC102 and to evaluate any change in hearing thresholds in healthy male and female subjects. RESULTS:The intratympanic delivery of AC102 thermogel was safe and well tolerated in healthy volunteers with normal hearing, with no permanent adverse events recorded. A mild and temporary volume-dependent conductive hearing loss in the higher frequencies was observed irrespective of AC102 or placebo, which did not result in long-term changes in hearing. Other transient adverse events related to the injection procedure were largely similar between placebo gel and AC102 suspension, consisting mainly of mild ear discomfort, tinnitus, otalgia, and the formation of a droplet of blood at the injection site. CONCLUSIONS:The results of this phase 1 clinical trial suggest that the intratympanic delivery of AC102 is a safe and well-tolerated approach for drug delivery to the inner ear in healthy volunteers with normal hearing. As temporary, volume-dependent, conductive hearing losses were observed in the higher frequencies, it is recommended that patients are counseled on a short-term increase in hearing thresholds following injection for conditions such as sudden hearing loss.

Aims: Clomipramine (CLOMI) has shown effectiveness in treating premature ejaculation but is linked to erectile dysfunction and reduced libido. Yohimbine (YOH), by contrast, is effective in treating erectile dysfunction and may improve libido. Combining CLOMI and YOH could potentially leverage the benefits of both drugs. This study aimed to investigate the interactions between these drugs and to evaluate their safety profile. Methods: A prospective, open-labelled, single-centre, pharmacokinetic (PK) drug-drug interaction study was performed in 15 healthy male subjects. Single-dose and steady-state PK were investigated using noncompartmental analysis after mono- and combination therapy of the 2 orally applied drugs. Plasma sampling was performed at baseline, 0.5 (YOH), 1, 1.5 (YOH), 2, 3, 4, 5, 6, 8, 12 and 24 h (CLOMI). Differences in the area under the curve after multiple dosing (MD) were determined using an equivalence boundary of 80-125%. Results: The geometric mean ratio of the area under the curve up to 12 h for MD CLOMI (combination vs. monotherapy) was 112% (90% confidence interval: 104-120%), whereas for MD YOH this ratio was 137% (90% confidence interval: 112-168%). The study drugs were safe and well tolerated as mono- and combination therapy, with no major adverse events reported. Conclusion: A PK assessment of clomipramine and yohimbine indicated a clinically significant drug-drug interaction for MD YOH in combination with CLOMI. This might be explained by competitive, CLOMI-related inhibition of YOH metabolism, probably mediated by cytochrome P450 2D6. However, according to European Medicines Agency guidelines, the effect can be classified as interaction absent (<1,25 fold) or minor (>1.25-<2-fold). Given the complimentary mechanisms of action and the favourable safety profiles, the findings pave the way for future efficacy studies.

AbstractSt. John's wort (SJW) extract, a herbal medicine with antidepressant effects, is a potent inducer of intestinal and/or hepatic cytochrome P450 (CYP) enzymes and P‐glycoprotein (P‐gp), which can cause clinically relevant drug interactions. It is currently not known whether SJW can also induce P‐gp activity at the human blood–brain barrier (BBB), which may potentially lead to decreased brain exposure and efficacy of certain central nervous system (CNS)‐targeted P‐gp substrate drugs. In this study, we used a combination of positron emission tomography (PET) imaging and cocktail phenotyping to gain a comprehensive picture on the effect of SJW on central and peripheral P‐gp and CYP activities. Before and after treatment of healthy volunteers (n = 10) with SJW extract with a high hyperforin content (3–6%) for 12–19 days (1800 mg/day), the activity of P‐gp at the BBB was assessed by means of PET imaging with the P‐gp substrate [11C]metoclopramide and the activity of peripheral P‐gp and CYPs was assessed by administering a low‐dose phenotyping cocktail (caffeine, omeprazole, dextromethorphan, and midazolam or fexofenadine). SJW significantly increased peripheral P‐gp, CYP3A, and CYP2C19 activity. Conversely, no significant changes in the peripheral metabolism, brain distribution, and P‐gp‐mediated efflux of [11C]metoclopramide across the BBB were observed following the treatment with SJW extract. Our data suggest that SJW does not lead to significant P‐gp induction at the human BBB despite its ability to induce peripheral P‐gp and CYPs. Simultaneous intake of SJW with CNS‐targeted P‐gp substrate drugs is not expected to lead to P‐gp‐mediated drug interactions at the BBB.

BACKGROUND:Accurate pharmacokinetic modelling in PET necessitates measurements of an input function, which ideally is acquired non-invasively from image data. For hepatic pharmacokinetic modelling two input functions need to be considered, to account for the blood supply from the hepatic artery and portal vein. Image-derived measurements at the portal vein are challenging due to its small size and image artifacts caused by respiratory motion. In this work we seek to demonstrate, using phantom experiments, how a dedicated PET/MR protocol can tackle these challenges and potentially provide input function measurements of the portal vein in a clinical setup. METHODS:A custom 3D printed PET/MR phantom was constructed to mimic the liver and portal vein. PET/MR acquisitions were made with emulated respiratory motion. The PET/MR imaging protocol consisted of high-resolution anatomical MR imaging of the portal vein, followed by a PET acquisition in parallel to a dedicated motion-tracking MR sequence. Motion tracking and deformation information were extracted from PET data and subsequently used in PET reconstruction to produce dynamic series of motion-free PET images. Anatomical MR images were used post PET reconstruction for partial volume correction of the input function measurements. RESULTS:Reconstruction of dynamic PET data with motion-compensation provided nearly motion-free series of PET frame data, suitable for image derived input function measurements of the portal vein. After partial volume correction, the individual input function measurements were within a 16.1% error range from the true activity in the portal vein compartment at the time of PET acquisition. CONCLUSION:The proposed protocol demonstrates clinically feasible PET/MR imaging of the liver for pharmacokinetic studies with accurate quantification of the portal vein input function, including correction for respiratory motion and partial volume effects.

Background and Hypothesis The dopamine theory of schizophrenia suggests that antipsychotics alleviate symptoms by blocking dopamine D2/3 receptors, yet a significant subset of patients does not respond adequately to treatment. To investigate potential predictors, we evaluated d-amphetamine-induced dopamine release and 1-year clinical outcomes in 21 antipsychotic-naive patients with first-episode schizophrenia.Study Design Twenty-one antipsychotic-naive patients (6 female) underwent dopamine D2/3 receptor radioligand [11C]-(+)-PHNO positron emission tomography. For estimating dopamine release, scans were performed with and without d-amphetamine pretreatment. The Positive and Negative Syndrome Scale was performed at regular intervals over 1 year while receiving treatment in a naturalistic setting (Clinical Trial Registry: EUDRACT 2010-019586-29).Study Results A group analysis revealed no significant differences in d-amphetamine-induced dopamine release between patients with or without clinically significant improvement. However, d-amphetamine-induced dopamine release in ventral striatum was significantly associated with reductions in positive symptoms (r = 0.54, P = .04; uncorrected P-values); release in globus pallidus correlated with a decrease in PANSS negative (r = 0.58, P = .02), general (r = 0.53, P = .04), and total symptom scores (r = 0.063, P = .01). Higher dopamine release in substantia nigra/ventral tegmental area predicted larger reductions in general symptoms (r = 0.51, P = .05). Post-amphetamine binding in putamen correlated positively with negative symptom scores at baseline (r = 0.66, P = .005) and throughout all follow-up visits.Conclusions These exploratory results support a relationship between d-amphetamine-induced dopamine release and the severity and persistence of symptoms during the first year of psychosis.

Background APN01 is a soluble recombinant human angiotensin-converting enzyme 2 (rhACE2), a key player in the renin–aldosterone–angiotensin system (RAAS). In clinical studies, APN01 was administered intravenously only, so far. The aim of this study (ClinicalTrials.gov: NCT05065645) was to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of inhaled APN01. Methods This was a phase I, double-blind, placebo-controlled, dose-escalation study. Inhalation was conducted via a nebuliser over 15 min in three single ascending dose (SAD) cohorts (n=24) and two multiple ascending dose (MAD) cohorts (n=16: every 12 h for 7 days). Doses in the SAD cohort were 1.25, 2.5 and 5 mg·mL−1; doses in the MAD cohort were 2.5 and 5 mg·mL−1. Safety (including adverse events (AEs), laboratory findings and lung function results), PK and PD data were assessed. Results In the SAD and MAD cohorts, treatment-related AEs were slightly more frequent in the active treatment group than in the placebo group. AEs were mild to moderate, with no dose-limiting toxicities. No clinically relevant changes in lung function and laboratory results were observed. The mean maximum observed plasma concentration (Cmax) values after single and multiple doses of 5 mg·mL−1 APN01 were 1.88 and 6.61 ng·mL−1, respectively. Among the PD variables, significance was found for ACE2 and angiotensin 1–5. Conclusions The application of aerosolised APN01 is safe and well tolerated after single and multiple doses. By achieving a high local concentration in the lungs and low systemic bioavailability, inhaled rhACE2 may present a therapeutic option in ACE2-related diseases.

Abstract Background P-glycoprotein (P-gp) is an efflux transporter which is abundantly expressed at the blood-brain barrier (BBB) and which has been implicated in the pathophysiology of various brain diseases. The radiolabelled antiemetic drug [11C]metoclopramide is a P-gp substrate for positron emission tomography (PET) imaging of P-gp function at the BBB. To assess whether [11C]metoclopramide can detect increased P-gp function in the human brain, we employed drug-resistant temporal lobe epilepsy (TLE) as a model disease with a well characterised, regional P-gp up-regulation at the BBB. Methods Eight patients with drug-resistant (DRE) TLE, 5 seizure-free patients with drug-sensitive (DSE) focal epilepsy, and 15 healthy subjects underwent brain PET imaging with [11C]metoclopramide on a fully-integrated PET/MRI system. Concurrent with PET, arterial blood sampling was performed to generate a metabolite-corrected arterial plasma input function for kinetic modelling. The choroid plexus was outmasked on the PET images to remove signal contamination from the neighbouring hippocampus. Using a brain atlas, 10 temporal lobe sub-regions were defined and analysed with a 1-tissue-2-rate constant compartmental model to estimate the rate constants for radiotracer transfer from plasma to brain (K1) and from brain to plasma (k2), and the total volume of distribution (VT = K1/k2). Results DRE patients but not DSE patients showed significantly higher k2 values and a trend towards lower VT values in several temporal lobe sub-regions located ipsilateral to the epileptic focus as compared to healthy subjects (k2: hippocampus: +34%, anterior temporal lobe, medial part: +28%, superior temporal gyrus, posterior part: +21%). Conclusions [11C]Metoclopramide PET can detect a seizure-induced P-gp up-regulation in the epileptic brain. The efflux rate constant k2 seems to be the most sensitive parameter to measure increased P-gp function with [11C]metoclopramide. Our study provides evidence that disease-induced alterations in P-gp expression at the BBB can lead to changes in the distribution of a central nervous system-active drug to the human brain, which could affect the efficacy and/or safety of drugs. [11C]Metoclopramide PET may be used to assess or predict the contribution of increased P-gp function to drug resistance and disease pathophysiology in various brain diseases. Trial registration EudraCT 2019-003137-42. Registered 28 February 2020.

PURPOSE:P-glycoprotein (P-gp) has been hypothesized to be involved in drug-resistance of epilepsy by actively extruding antiseizure medications (ASMs) from the brain. The P-gp inhibitor tariquidar (TQD) has been shown to effectively inhibit P-gp at the human blood-brain barrier, improving brain entry of several ASMs. A potential strategy to overcome drug-resistance is the co-administration of P-gp inhibitors such as TQD to ASMs. Here we present data on the tolerability of single-dose TQD as a potential add-on medication to ASMs. METHODS:We performed a multi-centre cohort study including drug-resistant epilepsy patients and healthy controls from the United Kingdom and Austria. TQD was administered intravenously at five different doses (2 mg/kg or 3 mg/kg of TQD were given to drug-resistant epilepsy patients and healthy controls, higher doses of TQD at 4 mg/kg, 6 mg/kg and 8 mg/kg as well as a prolonged infusion aiming at a dose of 6 mg/kg were only given to healthy controls). Adverse events were recorded and graded using the Common Terminology Criteria (CTCAE) scale. Additionally, TQD plasma concentration levels were measured and compared between drug-resistant patients and healthy controls. RESULTS:In total, 108 participants received TQD once at variable doses and it was overall well tolerated. At doses of 2 or 3 mg/kg TQD, only two of the 19 drug-resistant epilepsy patients and a third of the healthy controls (n = 14/42) reported adverse events probably related to TQD. The majority of those adverse events (96 %) were reported as mild. One drug-resistant epilepsy patient reported adverse events 24-hours after TQD administration possibly related to TQD-induced increased ASMs levels in the brain. CONCLUSIONS:TQD is an effective and well tolerated P-gp inhibitor as a single dose and could potentially be used intermittently in conjunction with ASMs to improve efficacy. This promising strategy to overcome drug-resistance in epilepsy should be investigated further in clinical randomised controlled trials.

Transporter-mediated drug-drug interactions (DDIs) are of concern in antimicrobial drug development, as they can have serious safety consequences. We used positron emission tomography (PET) imaging-based pharmacokinetic (PK) analysis to assess the effect of different drugs, which may cause transporter-mediated DDIs, on the tissue distribution and excretion of [ 18 F]ciprofloxacin as a radiolabeled model antimicrobial drug.

PET imaging using [11C]metoclopramide revealed the importance of P-glycoprotein (P-gp, ABCB1) in mediating the brain-to-blood efflux of substrates across the blood–brain barrier (BBB). In this work, the elimination rate constant from the brain (kE,brain), calculated from dynamic PET images without the need for arterial blood sampling, was evaluated as an outcome parameter for the interpretation of [11C]metoclopramide PET data. kE,brain parameter was obtained by linear regression of log-transformed brain time-activity curves (TACs). kE,brain values (h−1) obtained under baseline conditions were compared with values obtained after complete P-gp inhibition using tariquidar in rats (n = 4) and baboons (n = 4) or after partial inhibition using cyclosporine A in humans (n = 10). In baboons, the sensitivity of kE,brain to measure complete P-gp inhibition was compared with outcome parameters derived from kinetic modeling using a 1-tissue compartment model (1-TCM). Finally, kE,brain-maps were generated in each species using PMOD software. The linear part of the log-transformed brain TACs occurred from 10 to 30 min after radiotracer injection in rats, from 15 to 60 min in baboons, and from 20 to 60 min in humans. P-gp inhibition significantly decreased kE,brain values by 39 ± 12

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are two ATP-binding cassette efflux transporters that are coexpressed at the human blood–brain barrier (BBB) and blood–retina barrier (BRB). While pharmacological inhibition of P-gp and/or BCRP results in increased brain distribution of dual P-gp/BCRP substrate drugs, such as the tyrosine kinase inhibitor erlotinib, the effect of P-gp and/or BCRP inhibition on the retinal distribution of such drugs has hardly been investigated. In this study, we used positron emission tomography (PET) imaging to assess the effect of transporter inhibition on the distribution of [11C]erlotinib to the human retina and brain. Twenty two healthy volunteers underwent two PET scans after intravenous (i.v.) injection of a microdose (<5 μg) of [11C]erlotinib, a baseline scan, and a second scan either with concurrent i.v. infusion of tariquidar to inhibit P-gp (n = 5) or after oral intake of single ascending doses of erlotinib (300 mg, 650 mg, or 1000 mg, n = 17) to saturate erlotinib transport. In addition, transport of [3H]erlotinib to the retina and brain was assessed in mice by in situ carotid perfusion under various drug transporter inhibition settings. In comparison to the baseline PET scan, coadministration of tariquidar or erlotinib led to a significant decrease of [11C]erlotinib total volume of distribution (VT) in the human retina by −25 ± 8% (p ≤ 0.05) and −41 ± 16% (p ≤ 0.001), respectively. In contrast, erlotinib intake led to a significant increase in [11C]erlotinib VT in the human brain (+20 ± 16%, p ≤ 0.001), while administration of tariquidar did not result in any significant changes. In situ carotid perfusion experiments showed that both P-gp and BCRP significantly limit the distribution of erlotinib to the mouse retina and brain but revealed a similar discordant effect at the mouse BRB and BBB following co-perfusion with tariquidar and erlotinib as in humans. Co-perfusion with prototypical inhibitors of solute carrier transporters did not reveal a significant contribution of organic cation transporters (e.g., OCTs and OCTNs) and organic anion-transporting polypeptides (e.g., OATP2B1) to the retinal and cerebral distribution of erlotinib. In conclusion, we observed a dissimilar effect after P-gp and/or BCRP inhibition on the retinal and cerebral distribution of [11C]erlotinib. The exact mechanism for this discrepancy remains unclear but may be related to the function of an unidentified erlotinib uptake carrier sensitive to tariquidar inhibition at the BRB. Our study highlights the great potential of PET to study drug distribution to the human retina and to assess the functional impact of membrane transporters on ocular drug distribution.

The herpes simplex virus type 1 (HSV-1) mutant in 1814 contains an insertion mutation in the coding sequence for the virion transactivator protein VP16 and is thus impaired for the activation of immediate early (IE) gene expression. This virus was modified further by introducing the Moloney murine leukemia virus LTR promoter in place of the upstream sequences controlling expression of the IE regulatory protein ICPO, to yield mutant in 1820. In almost all cell types tested, in 1820 initiated infection less efficiently than in 1814, behaving as if lacking both VP16 and ICPO functions, but in BHK cells in 1820 was less impaired than in 1814. A rescuant of in 1820 at the VP16 locus, in 1825, also exhibited a host range phenotype, initiating replication as efficiently as wild-type HSV-1 in BHK cells but inefficiently in other cell types. In 1825 was unable to complement an ICPO null mutant in restricted cells, demonstrating that the promoter exchange prevented the expression of ICPO protein in functionally significant amounts. The novel host range properties of in 1820 provided a basis for the construction of additional viruses conditionally impaired for IE gene expression and assessment of their value as prototype vectors. Production of an HSV-1 mutant multiply defective in the expression of IE gene products was achieved by introduction of the temperature-sensitive mutation of HSV-1 tsK, which inactivates the IE transcription activator ICP4 at nonpermissive temperatures, into in 1820 to produce in 1820K. This mutant could be propagated effectively in BHK cells at 31 degrees but was effectively devoid of the major regulators ICPO, ICP4, and VP16 in other cells types at 38.5 degrees. Cultures could withstand infection with 5 PFU of in 1820K per cell without detectable cytopathology and could be reseeded to form colonies at approximately 90% efficiency. A derivative of in 1820K containing the Escherichia coli lacZ gene controlled by the human cytomegalovirus (HCMV) major IE promoter expressed low but detectable levels of beta-galactosidase in almost all cells after infection of cultures at 5 PFU per cell and incubation at 38.5 degrees. Cultures infected with 5 PFU per cell of an in 1820K derivative expressing neomycin phosphotransferase (npt) controlled by the HCMV IE promoter were resistant to killing by the antibiotic G418 for up to 3 days, and cell survival correlated with the retention of functional levels of npt. Mutants based on in 1820K can thus express foreign gene products in virtually all cells in a culture under conditions in which cytotoxicity is eliminated, demonstrating that progressive reduction of IE gene expression is an important step in the design of HSV-1-derived vectors.