Drug Exposure In Tissues Research Articles

Prediction of tissue exposures of polymyxin-B, amikacin and sulbactam using physiologically-based pharmacokinetic modeling.

The combination antimicrobial therapy consisting of amikacin, polymyxin-B, and sulbactam demonstrated in vitro synergy against multi-drug resistant Acinetobacter baumannii. The objectives were to predict drug disposition and extrapolate their efficacy in the blood, lung, heart, muscle and skin tissues using a physiologically-based pharmacokinetic (PBPK) modeling approach and to evaluate achievement of target pharmacodynamic (PD) indices against A. baumannii. A PBPK model was initially developed for amikacin, polymyxin-B, and sulbactam in adult subjects, and then scaled to pediatrics, accounting for both renal and non-renal clearances. The simulated plasma and tissue drug exposures were compared to the observed data from humans and rats. Efficacy was inferred using joint probability of target attainment of target PD indices. The simulated plasma drug exposures in adults and pediatrics were within the 0.5 to 2 boundary of the mean fold error for the ratio between simulated and observed means. Simulated drug exposures in blood, skin, lung, and heart were consistent with reported penetration ratio between tissue and plasma drug exposure. In a virtual pediatric population from 2 to <18 years of age using pediatric dosing regimens, the interpretive breakpoints were achieved in 85-90% of the population. The utility of PBPK to predict and simulate the amount of antibacterial drug exposure in tissue is a practical approach to overcome the difficulty of obtaining tissue drug concentrations in pediatric population. As combination therapy, amikacin/polymyxin-B/sulbactam drug concentrations in the tissues exhibited sufficient penetration to combat extremely drug resistant A. baumannii clinical isolates.

PK/PD Modeling to Inform Clinical Development of an Adeno-associated Virus Gene Transfer Therapy for Duchenne Muscular Dystrophy (S48.005)

<h3>Objective:</h3> To evaluate the pharmacokinetic (PK)/pharmacodynamic (PD) relationship between tissue vector genome exposure, biological efficacy and functional outcome in an animal model of Duchenne muscular dystrophy (DMD) (DMD^mdx mice) following treatment with delandistrogene moxeparvovec (SRP-9001). <h3>Background:</h3> Delandistrogene moxeparvovec is an investigational gene transfer therapy developed to address the root cause of DMD through targeted skeletal and cardiac muscle expression of SRP-9001 dystrophin protein, which contains key functional domains of dystrophin. <h3>Design/Methods:</h3> We evaluated vector biodistribution, expression, and clinical dose selection of delandistrogene moxeparvovec using a novel application of a PK and PD modeling approach applied to data collected from DMD^mdx mice. We analyzed PK/PD relationships between dose, tissue vector genome exposure, SRP-9001 dystrophin protein expression (percent dystrophin positive fibers [PDPF] and western blot), and functional improvement (relative specific force from tibialis anterior and diaphragm). <h3>Results:</h3> Linear kinetics with a dose-proportional increase in tissue drug exposure were demonstrated across the nearly 10-fold dose range (4.43×10¹³–4.01×10¹⁴ vg/kg), and in all tissues. The relationship between tissue vector exposure and PD endpoints (PDPF, motor function outcome) showed a saturable response across a wide range of vector exposures. The vector exposure at 1.33×10¹⁴ vg/kg (the clinical dose) approached the maximum treatment response. Relative specific force and PDPF were significantly correlated (<i>P</i>=4.43×10⁻⁶). However, the relationship appeared to be nonlinear, with increased PDPF expression approaching the maximal functional improvement. Relative specific force and western blot were not significantly correlated. <h3>Conclusions:</h3> For the first time, biodistribution, biomarker and functional efficacy data were used to quantify and demonstrate PK/PD relationships for an adeno-associated virus (AAV)-based gene transfer therapy in a DMD animal model. The results continue to support the expected therapeutic benefit and clinical dose of delandistrogene moxeparvovec, an AAV-based gene transfer therapy. <b>Disclosure:</b> Mrs. East has received personal compensation for serving as an employee of Sarepta Therapeutics. Rachael Potter has received stock or an ownership interest from Sarepta Therapeutics. Dr. Snedeker has received personal compensation for serving as an employee of Sarepta Therapeutics. Dr. Snedeker has stock in Sarepta Therapeutics. Dr. Haile has nothing to disclose. Christopher Wier has received personal compensation for serving as an employee of Sarepta Therapeutics. Christopher Wier has stock in Sarepta Therapeutics. Ms. Rodino-Klapac has received personal compensation for serving as an employee of Sarepta Therapeutics, Inc.. Ms. Rodino-Klapac has received stock or an ownership interest from Sarepta Therapeutics. Ms. Rodino-Klapac has received intellectual property interests from a discovery or technology relating to health care.

Feasibility Study to Assess Canagliflozin Distribution and Sodium-Glucose Co-Transporter 2 Occupancy Using [18 F]Canagliflozin in Patients with Type 2 Diabetes.

Sodium-glucose co-transporter 2 (SGLT2) inhibitors, including canagliflozin, reduce the risk of cardiovascular and kidney outcomes in patients with and without type 2 diabetes, albeit with a large interindividual variation. The underlying mechanisms for this variation in response might be attributed to differences in SGLT2 occupancy, resulting from individual variation in plasma and tissue drug exposure and receptor availability. We performed a feasibility study for the use of [18 F]canagliflozin positron emission tomography (PET) imaging to determine the association between clinical canagliflozin doses and SGLT2 occupancy in patients with type 2 diabetes. We obtained two 90-minute dynamic PET scans with diagnostic intravenous [18 F]canagliflozin administration and a full kinetic analysis in 7 patients with type 2 diabetes. Patients received 50, 100, or 300 mg oral canagliflozin (n = 2:4:1) 2.5 hours before the second scan. Canagliflozin pharmacokinetics and urinary glucose excretion were measured. The apparent SGLT2 occupancy was derived from the difference between the apparent volume of distribution of [18 F]canagliflozin in the baseline and post-drug PET scans. Individual canagliflozin area under the curve from oral dosing until 24-hours (AUCP0-24h ) varied largely (range 1,715-25,747 μg/L*hour, mean 10,580 μg/L*hour) and increased dose dependently with mean values of 4,543, 6,525, and 20,012 μg/L*hour for 50, 100, and 300 mg, respectively (P = 0.046). SGLT2 occupancy ranged between 65% and 87%, but did not correlate with canagliflozin dose, plasma exposure, or urinary glucose excretion. We report the feasibility of [18 F]canagliflozin PET imaging to determine canagliflozin kidney disposition and SGLT2 occupancy. This suggests the potential of [18 F]canagliflozin as a tool to visualize and quantify clinically SGLT2 tissue binding.

A Non-radiometric Approach to Determine Tissue Vascular Blood Volume in Biodistribution Studies.

The aim of this research was to develop a reliable non-radiometric method to measure the residual blood in tissue without the need for perfusion or radiometric measurements in biodistribution studies. It was found that the perfusion method not only was ineffective in removing blood from tissue, but also introduced additional variability in the determination of tissue drug exposure and was not reproducible across studies. In addition, the use of hemoglobin as an endogenous protein and biomarker for tissue blood content was studied and it was found that hemoglobin measurement in tissue was not a reliable and effective approach for determination of residual blood level in tissue. To evaluate an alternative method for addressing the tissue blood level in biodistribution studies, animals were dosed with a Residual Blood Determinant Reagent (RBDR) 5min prior to tissue harvesting. The level of RBDR, an exogenous protein, was measured in whole blood homogenate and in tissue lysate. Based on the level of the RBDR, the vascular blood volume (VBV) in tissue was calculated and then the tissue exposures were corrected based on the blood volumes. The tissue VBVs measured by the RBDR method were comparable with the literature values obtained by radiometric measurements.

VP.82 PK/PD modelling to inform clinical development of an adeno-associated virus gene transfer therapy for Duchenne muscular dystrophy

Delandistrogene moxeparvovec (SRP-9001) is an investigational gene transfer therapy for Duchenne muscular dystrophy (DMD) developed for targeted skeletal and cardiac muscle expression of micro-dystrophin—a shortened, functional dystrophin protein. We evaluated vector biodistribution, expression, and clinical dose selection of delandistrogene moxeparvovec using a novel application of a pharmacokinetic (PK) and pharmacodynamic (PD) modelling approach applied to data collected from an animal model of DMD (DMD^MDX mice). We analysed PK/PD relationships between dose, tissue vector genome exposure, micro-dystrophin protein expression (percentage dystrophin positive fibres [PDPF] and western blot), and functional improvement (relative specific force from tibialis anterior and diaphragm). Linear kinetics with a dose-proportional increase in tissue drug exposure was demonstrated across the nearly 10-fold dose range (4.43x10¹³–4.01x10¹⁴ vg/kg), and in all tissues. The relationship between tissue vector exposure and PD endpoints (PDPF, motor function outcome) showed a saturable response across a wide range of vector exposures. The vector exposure at 1.33x10¹⁴ vg/kg (the clinical dose) approached the maximum treatment response. Relative specific force and PDPF were significantly correlated (P=4.43x10^-6). However, the relationship appeared to be nonlinear, with increased PDPF expression approaching the maximal functional improvement. Relative specific force and western blot were not significantly correlated. For the first time, biodistribution, biomarker, and functional efficacy data were used to quantify and demonstrate PK/PD relationships for an adeno-associated virus (AAV)-based gene transfer therapy in a DMD animal model. The results continue to support the expected therapeutic benefit and clinical dose of delandistrogene moxeparvovec, an AAV-based gene transfer therapy.

Studying Telmisartan Plasma Exposure, Kidney Distribution, Receptor Occupancy, and Response in Patients With Type 2 Diabetes Using [11 C]Telmisartan.

The angiotensin receptor blocker telmisartan slows progression of kidney disease in patients with type 2 diabetes (T2D), yet many patients remain at high risk for progressive kidney function loss. The underlying mechanisms for this response variation might be attributed to differences in angiotensin-1 receptor occupancy (RO), resulting from individual variation in plasma drug exposure, tissue drug exposure, and receptor availability. Therefore, we first assessed the relationship between plasma telmisartan exposure and urinary-albumin-to-creatinine-ratio (UACR) in 10 patients with T2D and albuminuria (mean age 66 years, median UACR 297 mg/g) after 4 weeks treatment with 80 mg telmisartan once daily. Increasing telmisartan exposure associated with a larger reduction in UACR (Pearson correlation coefficient (PCC)=-0.64, P=0.046, median change UACR: -40.1%, 95% confidence interval (CI): -22.9 to -77.4%, mean telmisartan area under the curve (AUC)=2927.1 ng·hour/mL, 95% CI: 723.0 to 6501.6 ng·hour/mL). Subsequently, we assessed the relation among plasma telmisartan exposure, kidney distribution, and angiotensin-1 RO in five patients with T2D (mean age 60 years, median UACR 72 mg/g) in a separate positron emission tomography imaging study with [11 C]Telmisartan. Individual plasma telmisartan exposure correlated with telmisartan distribution to the kidneys (PCC=0.976, P=0.024). A meaningful RO could be calculated in three patients receiving 120 mg oral telmisartan, and although high exposure seems related to higher RO, with AUC0-last of 31, 840, and 274 ng·hour/mL and corresponding RO values 5.5%, 44%, and 59%, this was not significant (P=0.64). Together these results indicate, for the first time, a relationship among interindividual differences in plasma exposure, kidney tissue distribution, RO, and ultimately UACR response after telmisartan administration.

Real-world Endoscopic and Histological Outcomes Are Correlated with Ustekinumab Exposure in Patients with Ulcerative Colitis.

Histo-endoscopic outcomes are being proposed as new treatment targets in ulcerative colitis [UC]. Little is known about the pharmacokinetic-pharmacodynnamic [PK-PD] relationship of ustekinumab [UST] in UC patients or whether serum UST concentrations reflect tissue drug exposure. We aimed to study UST serum concentrations and their relation to tissue exposure and drug effectiveness in a real-world setting. A total of 42 UC patients starting UST were prospectively followed by clinical, endoscopic and histological assessments at Week 16. Histological remission was defined as Nancy Histology Index of 0. Analogous to the UNIFI programme, histo-endoscopic mucosal improvement was defined as a combination of histological improvement [Geboes ≤3.1] and endoscopic improvement [MES ≤1]. Paired trough serum samples and colonic mucosal biopsies were collected for UST levels measurement. After 16 weeks [IQR 15.8-16.4] of therapy, histological remission and histo-endoscopic mucosal improvement were observed in 19 [45%] and 18 [43%] patients, respectively. Patients who achieved these outcomes had higher serum UST levels than those who did not. Patients with shorter disease duration and clinical response at Week 8 had higher odds to achieve histological remission. UST concentrations from paired serum and biopsy samples revealed a strong positive correlation [r = 0.88, p < 0.001], in both inflamed and uninflamed tissue. In this real-world cohort of refractory UC patients initiating UST, more than a third of the patients achieved histological remission. A drug exposure-response relationship was observed for histo-endoscopic outcomes, with no added value of measuring tissue exposure given the strong correlation with serum exposure.

P442 Real-world endoscopic and histologic outcomes are linked to ustekinumab exposure in Ulcerative Colitis

Abstract Background Histologic healing is being proposed as new treatment target in ulcerative colitis (UC), and the concept of histo-endoscopic mucosal improvement was introduced in the UNIFI study with ustekinumab (UST) in moderate–to-severe UC. Very little is known about the Pk-PD relationship of ustekinumab in UC patients, and especially whether serum UST concentrations correlate well with colonic tissue drug exposure and with histologic healing. The aim of this study is to provide real-world data including histology, and to study UST serum concentrations and its relation to tissue levels and drug efficacy. Methods UC patients starting UST in standard dosage at our referral centre were prospectively followed by clinical and endoscopic assessments at week, 16 or week, 24, and colonic biopsies were taken for histopathologic scoring. Histologic remission was defined as Nancy histology index (NHI) of, 0. Other collected outcomes were clinical response, clinical remission, endoscopic improvement, endoscopic remission and mucosal healing (definitions in Figure, 1). Paired trough serum sample and colonic mucosal biopsy were collected for UST levels measurement using a CE marked in-house developed ELISA. Results A total of, 42 UC patients started ustekinumab between June, 2019 and May, 2021, allowing a follow-up of at least, 6 months (Table, 1). By week, 24, clinical response, clinical remission, endoscopic improvement, endoscopic remission, and mucosal healing were observed in, 31 (74%), 24 (57%), 22 (52%), 11 (26%) and, 10 (24%) patients, respectively (Figure, 1). Histologic remission was observed in, 19 (45%) patients, of whom, 10 and, 9 with endoscopic Mayo, 0 and, 1, respectively. Multivariate analysis identified clinical response at week, 8 as a predictor for histologic remission at week, 24 [OR, 8.84, p = 0.024], and inversely correlated with therapy discontinuation [OR, 0.10, p = 0.006]. A trend of higher UST serum trough levels was observed in patients achieving histologic and endoscopic outcomes in comparison with patients who did not reach these outcomes, with a significant statistical difference at week, 8 for endoscopic outcomes (Figure, 2). UST concentrations from paired serum and biopsy samples revealed a strong positive correlation (Spearman r=0.88, p&amp;lt;0.001, n=17), both in inflamed (mayo endoscopic score &amp;gt;1) (r=0.89, p&amp;lt;0.001, n=10) and uninflamed (mayo endoscopic score ≤, 1) tissue (r=0.88, p&amp;lt;0.008, n=7) (Figure, 3). Conclusion In this real-word cohort of UC patients initiating UST, more than a third of the patients achieved histologic remission. Serum UST levels were furthermore strongly correlated with tissue levels of UST. A drug exposure-response relationship was observed for histologic and endoscopic outcomes.

Therapeutic concentrations of calcineurin inhibitors do not deregulate glutathione redox balance in human renal proximal tubule cells.

The calcineurin inhibitors (CNI) cyclosporine A and tacrolimus comprise the basis of immunosuppressive regimes in all solid organ transplantation. However, long-term or high exposure to CNI leads to histological and functional renal damage (CNI-associated nephrotoxicity). In the kidney, proximal tubule cells are the only cells that metabolize CNI and these cells are believed to play a central role in the origin of the toxicity for this class of drugs, although the underlying mechanisms are not clear. Several studies have reported oxidative stress as an important mediator of CNI-associated nephrotoxicity in response to CNI exposure in different available proximal tubule cell models. However, former models often made use of supra-therapeutic levels of tissue drug exposure. In addition, they were not shown to express the relevant enzymes (e.g., CYP3A5) and transporters (e.g., P-glycoprotein) for the metabolism of CNI in human proximal tubule cells. Moreover, the used methods for detecting ROS were potentially prone to false positive results. In this study, we used a novel proximal tubule cell model established from human allograft biopsies that demonstrated functional expression of relevant enzymes and transporters for the disposition of CNI. We exposed these cells to CNI concentrations as found in tissue of stable solid organ transplant recipients with therapeutic blood concentrations. We measured the glutathione redox balance in this cell model by using organelle-targeted variants of roGFP2, a highly sensitive green fluorescent reporter protein that dynamically equilibrates with the glutathione redox couple through the action of endogenous glutaredoxins. Our findings provide evidence that CNI, at concentrations commonly found in allograft biopsies, do not alter the glutathione redox balance in mitochondria, peroxisomes, and the cytosol. However, at supra-therapeutic concentrations, cyclosporine A but not tacrolimus increases the ratio of oxidized/reduced glutathione in the mitochondria, suggestive of imbalances in the redox environment.

Distribution of 14C-Latanoprost Following a Single Intracameral Administration Versus Repeated Topical Administration.

PurposeTo qualitatively evaluate the ocular and periocular distribution of 14C-latanoprost following a single intracameral administration or repeated topical ocular administration in beagle dogs and cynomolgus monkeys.MethodsIn the dog study, three animals received an intracameral dose of 14C-latanoprost bilaterally and were euthanized at 1, 2, and 4 h post dose; three control animals received topical 14C-latanoprost bilaterally once daily for 5 days and were euthanized at 1, 4, and 24 h post final dose. Sagittal 40-µm sections of eyes with surrounding tissues were collected and processed for autoradiography. Methods in the monkey study were similar; two animals received a unilateral intracameral dose of 14C-latanoprost.ResultsAfter intracameral dosing in dogs, radioactivity was concentrated in the cornea, iris, ciliary body, and anterior chamber with no radioactivity detected in the eyelids or other periorbital tissues. After topical dosing, radioactivity was distributed in the bulbar conjunctiva, cornea, anterior chamber, iris, ciliary body, upper and lower eyelids, and periorbital tissues (fat/muscle). After intracameral dosing in monkeys, radioactivity was concentrated in the anterior chamber, cornea, iris, ciliary body, and posteriorly along the uveoscleral outflow pathway; there was no radioactivity in the eyelids or periorbital tissues aside from signal in the nasolacrimal duct, likely from reflux of 14C-latanoprost into the tear film.ConclusionsIntracameral delivery resulted in more selective target tissue drug exposure. Intracameral drug delivery has potential to reduce ocular surface and periocular adverse effects associated with topical administration of prostaglandin analogues, such as eyelash growth and periorbital fat atrophy.Electronic supplementary materialThe online version of this article (10.1007/s40123-020-00285-3) contains supplementary material, which is available to authorized users.

Translational Approach to Predicting the Efficacy of Maraviroc-Based Regimens as HIV Preexposure Prophylaxis.

Maraviroc-based regimens have been explored as preexposure prophylaxis (PrEP) against human immunodeficiency virus (HIV). In this study, we utilized mucosal tissue drug exposure data, combined with target concentrations generated in vitro, in a pharmacokinetic-pharmacodynamic analysis to predict the effects of drug combinations and adherence on PrEP efficacy. Mucosal tissue concentrations of maraviroc were measured in 24 healthy women. The 90% effective concentrations (EC90) of maraviroc (alone and combined with tenofovir and emtricitabine) for protection against HIV were identified in CD4+ T cells. Monte Carlo simulations were performed to identify dosing strategies to protect colorectal and female genital tract (FGT) tissues from HIV infection. Colorectal maraviroc concentrations were 350-fold higher than in the FGT. Under steady-state conditions, our model predicted that one 300-mg dose/week was sufficient to protect colorectal tissue from HIV in 99% of the population, while 300 mg daily would protect the FGT in only 63% of the population. FGT protection increased to >90% when maraviroc was used in combination with tenofovir (5 doses/week) or emtricitabine (3 doses/week). Poor adherence resulted in a drastic decrease in efficacy in the FGT but not colorectal tissue. However, greater forgiveness was seen when maraviroc was combined with tenofovir or emtricitabine, suggesting that maraviroc should not be used alone as PrEP.

Plasma and tissue pharmacokinetics of fosfomycin in morbidly obese and non-obese surgical patients: a controlled clinical trial

Objectives To assess the pharmacokinetics and tissue penetration of fosfomycin in obese and non-obese surgical patients. Methods Fifteen obese patients undergoing bariatric surgery and 15 non-obese patients undergoing major intra-abdominal surgery received an intravenous single short infusion of 8 g of fosfomycin. Fosfomycin concentrations were determined by LC-MS/MS in plasma and microdialysate from subcutaneous tissue up to 8 h after dosing. The pharmacokinetic analysis was performed in plasma and interstitial fluid (ISF) by non-compartmental methods. Results Thirteen obese patients (BMI 38–50 kg/m2) and 14 non-obese patients (BMI 0–29 kg/m2) were evaluable. The pharmacokinetics of fosfomycin in obese versus non-obese patients were characterized by lower peak plasma concentrations (468 ± 139 versus 594 ± 149 mg/L, P = 0.040) and higher V (24.4 ± 6.4 versus 19.0 ± 3.1 L, P = 0.010). The differences in AUC∞ were not significant (1275 ± 477 versus 1515 ± 352 mg·h/L, P = 0.16). The peak concentrations in subcutaneous tissue were reached rapidly and declined in parallel with the plasma concentrations. The drug exposure in tissue was nearly halved in obese compared with non-obese patients (AUC∞ 1052 ± 394 versus 1929 ± 725 mg·h/L, P = 0.0010). The tissue/plasma ratio (AUCISF/AUCplasma) was 0.86 ± 0.32 versus 1.27 ± 0.34 (P = 0.0047). Conclusions Whereas the pharmacokinetics of fosfomycin in plasma of surgical patients were only marginally different between obese and non-obese patients, the drug exposure in subcutaneous tissue was significantly lower in the obese patients.

Long-Acting Profile of 4 Drugs in 1 Anti-HIV Nanosuspension in Nonhuman Primates for 5 Weeks After a Single Subcutaneous Injection

Daily oral antiretroviral therapy regimens produce limited drug exposure in tissues where residual HIV persists and suffer from poor patient adherence and disparate drug kinetics, which all negatively impact outcomes. To address this, we developed a tissue- and cell-targeted long-acting 4-in-1 nanosuspension composed of lopinavir (LPV), ritonavir, tenofovir (TFV), and lamivudine (3TC). In 4 macaques dosed subcutaneously, drug levels over 5 weeks in plasma, lymph node mononuclear cells (LNMCs), and peripheral blood mononuclear cells (PBMCs) were analyzed by liquid chromatography–tandem mass spectrometry. Plasma and PBMC levels of the active drugs (LPV, TFV, and 3TC) were sustained for 5 weeks; PBMC exposures to LPV, ritonavir, and 3TC were 12-, 16-, 42-fold higher than those in plasma. Apparent T1/2z of LPV, TFV, and 3TC were 219.1, 63.1, and 136.3 h in plasma; 1045.7, 105.9, and 127.7 h in PBMCs. At day 8, LPV, TFV, and 3TC levels in LNMCs were 4.1-, 5.0-, and 1.9-fold higher than in those in PBMCs and much higher than in plasma. Therefore, 1 dose of a 4-drug nanosuspension exhibited persistent drug levels in LNMCs, PBMCs, and plasma for 5 weeks. With interspecies scaling and dose adjustment, this 4-in-1 HIV drug-combination could be a long-acting treatment with the potential to target residual virus in tissues and improve patient adherence.

Are the Present Doses of Anti Tubercular Drugs Adequate for Severe Disease?

Are the Present Doses of Anti Tubercular Drugs Adequate for Severe Disease?

Quantitative Whole Body Autoradiography (QWBA) Analysis Reveals That CPX-351 Shifts the Exposure of Cytarabine (Cyt) and Daunorubicin (Daun) Away from Many Tissues While Providing Prolonged Exposure to Cytotoxic Drug Concentrations in the Bone Marrow Compared to Conventional Free Drug Administration

Background: CPX-351 is a liposomal formulation co-encapsulating Cyt and Daun, that delivers the drugs in vivo at a 5:1 molar ratio shown to be synergistic preclinically. Clinically, CPX-351 has provided evidence of promising improvements in patient outcomes, most notably in elderly newly diagnosed high risk (secondary) AML and in unfavorable risk first relapse adult AML where statistically significant increases in overall survival where observed in two randomized, controlled Phase 2 studies. In patients, CPX-351 displays a volume of distribution equal to the plasma volume and first order elimination with a half-life of > 24h for both drugs while maintaining the circulating Cyt:Daun molar ratio near 5:1. This is in contrast to the two drugs as conventionally administered in non-liposomal (NL) aqueous solution form where very rapid drug elimination is observed. Comparison of tissue distribution over time between CPX-351 and NL Cyt:Daun was performed in rats to better understand the pharmacodynamic relationships for CPX-351 in the context of what is known for the non-liposomal (NL) drugs, particularly as it relates to tissues relevant to efficacy and drug toxicity.Methods: Duplicate batches utilizing either [14C]Daun or [14C]Cyt CPX-351 or saline solutions of like labeled Cyt+Daun were prepared. Long-Evans rats received single IV bolus doses of either 15 units/kg (15 mg/kg Cyt + 6.64 mg/kg Daun) CPX-351, or a saline solution of 300 mg/kg Cyt + 10 mg/kg Daun. These doses were selected based on allometric scaling to reflect the respective clinical doses of CPX-351 and non-infusional Cyt:Daun treatment regimens. Animals were sacrificed at the designated time points post-dose and were frozen in a dry-ice/hexane bath in preparation for QWBA procedures. The tissue distribution of test article-derived radioactivity was determined using QWBA. Exposure of Cyt and Daun to a wide range of tissues was estimated based on the tissue density of [14C]Cyt-derived or [14C]Daun-derived radioactivity from QWBA section images. Tissue drug exposure comparisons between CPX-351 and NL Cyt:Daun were performed using Cmax and AUC0-t values.Results: The rapid tissue distribution of Cyt and Daun following injection of the NL form of the combination was reflected by the recovery of <5% of either drug in the plasma 15 minutes after injection and tissue/plasma AUC0-t ratios that were >1 and >10 for [14C]Cyt and [14C]Daun, respectively, for the majority of tissues studied. In contrast, for CPX-351 virtually all of the injected Cyt and Daun was present in the plasma between 0.25-1.0 hours and corresponding tissue/plasma AUC0-t ratios in the majority of tissues were <0.05 and <0.2 for [14C]Cyt and [14C]Daun, respectively. These differences were readily apparent in the QWBA section images. For the NL form of the combination, [14C]Cyt and [14C]Daun were widely distributed throughout the body shortly after injection. Following CPX-351 administration, radioactivity was more limited to discreet tissues and organs. Distribution of Cyt into tissues after CPX-351 administration was reduced as well as much slower than after NL Cyt injection as reflected by markedly lower Cmax values in all non-vascular tissues as well as lower AUC values in a majority of tissues. Comparing [14C]Daun distribution for CPX-351 vs NL Daun revealed a slower removal from the blood/plasma compartment and gradual distribution to tissues for CPX-351 with a similar general tissue profile as for NL drug with the notable increases in exposure to spleen, liver, testis and bone marrow. Bone marrow levels of Cyt and Daun peaked at 24h post CPX-351 injection and persisted for several days at anti-leukemic concentrations; drug levels present in the marrow at 96h were well above the CPX-351 IC50 values previously observed with fresh AML patient blast samples. In contrast, bone marrow Cyt concentrations fell below detectable limits within 24h after administration of NL drug.Conclusions: CPX-351 shifts the exposure of Cyt and Daun away from most non-hematologic tissues compared to NL drug treatment. Importantly, CPX-351 accumulates and persists in the bone marrow for over 4 days at concentrations known to have anti-leukemic activity against AML blasts. Taken together, these results provide additional biologic rationale that support the clinical improvements in both efficacy and safety seen for CPX-351 in randomized trials compared to conventional Cyt + Daun treatment. DisclosuresMayer:Celator: Employment, Equity Ownership, Patents & Royalties. Sadowski:Xenobiotic Laboratories: Employment. Tardi:Celator Pharmaceuticals: Employment, Equity Ownership. Xie:Celator Pharmaceuticals: Employment, Equity Ownership. Cabral-Lilly:Ceator Pharmaceuticals: Employment, Equity Ownership. Heller:Xenobiotic Laboratories: Employment, Research Funding.

Importance of Relating Efficacy Measures to Unbound Drug Concentrations for Anti-Infective Agents

For the optimization of dosing regimens of anti-infective agents, it is imperative to have a good understanding of pharmacokinetics (PK) and pharmacodynamics (PD). Whenever possible, drug efficacy needs to be related to unbound concentrations at the site of action. For anti-infective drugs, the infection site is typically located outside plasma, and a drug must diffuse through capillary membranes to reach its target. Disease- and drug-related factors can contribute to differential tissue distribution. As a result, the assumption that the plasma concentration of drugs represents a suitable surrogate of tissue concentrations may lead to erroneous conclusions. Quantifying drug exposure in tissues represents an opportunity to relate the pharmacologically active concentrations to an observed pharmacodynamic parameter, such as the MIC. Selection of an appropriate specimen to sample and the advantages and limitations of the available sampling techniques require careful consideration. Ultimately, the goal will be to assess the appropriateness of a drug and dosing regimen for a specific pathogen and infection.

Ablation of Both Organic Cation Transporter (Oct)1 and Oct2 Alters Metformin Pharmacokinetics but Has No Effect on Tissue Drug Exposure and Pharmacodynamics

Organic cation transporter (OCT)1 and OCT2 mediate hepatic uptake and secretory renal clearance of metformin, respectively. Pharmacokinetic/pharmacodynamic (PK/PD) implications of simultaneous impairment of both transporters, such as by systemic pan-OCT inhibition, have not been studied directly. At present metformin PK/PD, distribution, and excretion were studied in Oct1/Oct2-knockout mice. Metformin clearance was reduced 4.5-fold from renal blood flow to unbound glomerular filtration rate, and volume of distribution was reduced 3.5-fold in Oct1/Oct2-knockout mice. Oral bioavailability was not affected (F = 64 ± 4 versus 59 ± 11; knockout versus wild type). Liver- and kidney-to-plasma concentration ratios were decreased in Oct1/Oct2-knockout mice 4.2- and 2.5-fold, respectively. The 2.9-fold increase in oral metformin exposure and reduced tissue partitioning yielded little to no net change in tissue drug concentrations. Absolute kidney exposure was unchanged (knockout/wild type = 1.1 ± 0.2), and liver exposure was only modestly decreased (knockout/wild type = 0.6 ± 0.1). Oral glucose area under the curve (AUC) lowering by metformin was not impaired in Oct1/Oct2-knockout mice at the five dose levels tested (ED50 = 151 versus 110 mg/kg; glucose lowering at highest dose = 42 ± 1 versus 39 ± 4%; knockout versus wild type); however, higher systemic metformin exposures were necessary in knockout mice to elicit the same effect (half-maximal efficacious AUC = 70 versus 26 μg x h/ml). Despite major changes in metformin clearance and volume of distribution in Oct1/Oct2-knockout mice, tissue drug exposure and PD were not affected. These findings challenge the presumption that systemic OCT inhibition will affect metformin pharmacology.

Enhanced Analgesic Responses After Preferential Delivery of Morphine and Fentanyl to the Olfactory Epithelium in Rats

Centrally acting opioid analgesics such as morphine and fentanyl are effective, but their efficacy is often limited by a delayed response or side effects resulting from systemic first pass before reaching the brain and the central nervous system (CNS). It is generally accepted that drugs applied to the nasal cavity can directly access the brain and the CNS, which could provide therapeutic advantages such as rapid onset and lower systemic exposure. The olfactory region of the nasal cavity has been implicated in facilitating this direct nose-to-CNS transfer. If the fraction of opioid administered to the olfactory region could be improved, there could be a larger fraction of drug directly delivered to the CNS, mediating greater therapeutic benefit. We have developed a pressurized olfactory delivery (POD) device to consistently and noninvasively deposit a majority of drug on the olfactory region of the nasal cavity in Sprague-Dawley rats. Using the tail-flick latency test and analysis of plasma and CNS tissue drug exposure, we compared distribution and efficacy of the opioids morphine and fentanyl administered to the nasal olfactory region with the POD device or the nasal respiratory region with nose drops or systemically via intraperitoneal injection. Compared with nose drop administration, POD administration of morphine resulted in a significantly higher overall therapeutic effect (area under the curve [over the time course] [AUC](effect)) without a significant increase in plasma drug exposure (AUC(plasma)). POD of morphine resulted in a nose-to-CNS direct transport percentage of 38% to 55%. POD of fentanyl led to a faster (5 vs 10 minutes) and more intense analgesic effect compared with nasal respiratory administration. Unlike intraperitoneal injection or nose drop administration, both morphine and fentanyl given by the POD device to olfactory nasal epithelium exhibited clockwise (plasma) versus effect hysteresis after nasal POD administration, consistent with a direct nose-to-CNS drug transport mechanism. Deposition of opioids to the olfactory region within the nasal cavity could have a significant impact on drug distribution and pharmacodynamic effect, and thus should be considered in future nasally administered opioid studies.

Penetration of antibacterials into bone: what do we really need to know for optimal prophylaxis and treatment of bone and joint infections?

The rationale for appropriate antimicrobial treatment of deep-seated infections has progressively changed over the last 10 years. Although the antimicrobial choice used to be essentially based only on the spectrum of activity and on the in vitro susceptibility of bacterial pathogens, it has become evident that at least two additional factors have to be borne in mind whenever bacteria are located within organs. In the presence of deep-seated infections, it is mandatory to achieve optimal pharmacodynamic exposure with antimicrobials not only in plasma but also at the infection site. This means that both the pharmacokinetic characteristics of the drugs and the anatomical characteristics of the infection sites must be taken into account in order to avoid drug underexposure in the various organs. Additionally, in critically ill patients, particular attention should be given to the possibility that the pathophysiological changes that frequently occur in this setting may significantly alter drug exposure at the infection site, especially for hydrophilic antimicrobials. Greater understanding of the type of antibacterial activity has emphasized the need to choose different dosages and dosing regimens for the various antibacterials. As a general rule, for time-dependent antimicrobials, (i.e. b-lactams, glycopeptides and oxazolidinones), administration of the total daily dosage in multiple doses or as a continuous infusion may be worthwhile, with the intent of favouring more sustained concentrations and thus maximizing the time during which the concentration persists above the minimum inhibitory concentration (T >MIC). Accordingly, in daily clinical practice, optimization of drug exposure with time-dependent antimicrobials may be pursued through maintenance of minimum (trough) plasma trough concentrations (Cmin) above the MIC (Cmin >MIC). Conversely, for concentration-dependent agents (i.e.fluoroquinolones and aminoglycosides), once-daily dosing should be chosen whenever feasible, with the intent of maximizing plasma drug exposure either in terms of the maximum plasma concentration (Cmax) [a Cmax/MIC ratio >10] or in terms of the total daily exposure over time (an area under the plasma concentrationtime curve [AUC]/MIC ratio >100). In fact, for these agents, unlike the time-dependent antimicrobials, higher concentrations mean more rapid and potent bacterial killing. In this context, plasma concentrations are usually considered to be valuable surrogate markers of drug exposure in tissue during deep-seated infections. In fact, it is usually expected that an equilibrium of drug concentrations between plasma and the tissue interstitium – namely, the site where bacterial pathogens aremost frequently located within organs – is achieved. However, it has been shown that plasma concentrations may not always correctly predict concentrations in the tissue interstitium. Drug penetration may be altered in the presence of peculiar pathophysiological conditions as, for example, has been observed in patients with septic shock. In these cases, dose adjustments that are exclusively based on plasma concentration data may therefore be misleading. Interestingly, direct measurement of drug concentrations at the infection site is gaining increasing relevance for appropriate treatment of deep-seated infections with antimicrobials, especially for sites where direct collection of extracellular fluids is feasible, e.g. by direct sampling of epithelial lining fluid in cases of pneumonia, cerebrospinal fluid in cases of meningitis or vitreous humour in cases of endophthalmitis. In some other tissue sites where direct sampling is not feasible, interstitial fluids may be collected by means of microdialysis, which is a probe-based sampling method employed to measure drug concentrations in selected tissues; for example, in the interstitium of muscle and the subcutis in the case of skin and softtissue infections. Conversely, whenever assessment of the extracellular drug concentration in tissues is not feasible either by direct sampling or by microdialysis, measurement is usually performed in tissue homogenates. Indeed, the relevance of findings from studies COMMENTARY Clin Pharmacokinet 2009; 48 (2): 125-127 0312-5963/09/0002-0125/$49.95/0

Stereoselective disposition of S‐ and R‐licarbazepine in mice

The stereoselective disposition of S-licarbazepine (S-Lic) and R-licarbazepine (R-Lic) was investigated in plasma, brain, liver, and kidney tissues after their individual administration (350 mg/kg) to mice by oral gavage. Plasma, brain, liver, and kidney concentrations of licarbazepine enantiomers and their metabolites were determined over the time by a validated chiral HPLC-UV method. The mean concentration data, attained at each time point, were analyzed using a non-compartmental model. S-Lic and R-Lic were rapidly absorbed from gastrointestinal tract of mouse and immediately distributed to tissues supplied with high blood flow rates. Both licarbazepine enantiomers were metabolized to a small extent, each parent compound being mainly responsible for the systemic and tissue drug exposure. The stereoselectivity in the metabolism and distribution of S- and R-Lic was easily identified. An additional metabolite was detected following R-Lic administration and S-Lic showed a particular predisposition for hepatic and renal accumulation. Stereoselective processes were also identified at the blood-brain barrier, with the brain exposure to S-Lic almost twice that of R-Lic. Another finding, reported here for the first time, was the ability of the mouse to perform the chiral inversion of S- and R-Lic, albeit to a small extent.