High First-pass Metabolism Research Articles

PXR-mediated induction of human CYP3A4 and mouse Cyp3a11 by the glucocorticoid budesonide

Budesonide, a glucocorticoid with a high first-pass metabolism, is used for the oral treatment of inflammatory bowel disease. Cytochrome P450 3A4 (CYP3A4) is an enzyme involved in the metabolism of numerous drugs, including budesonide. Since inhibition or induction of CYP3A4 is often the cause of drug–drug interactions we analyzed how budesonide affects the activity and expression of this enzyme. CYP3A4 activity was assessed by the metabolism of a luminogenic substrate (luciferin-benzylether) using recombinant human CYP3A4 protein. We observed no inhibition of the metabolism in the presence of budesonide at concentrations up to 25 μM. Induction experiments in human LS180 colon carcinoma cells showed an increased expression of CYP3A4 mRNA after budesonide treatment. Transactivation assays revealed that budesonide activates the CYP3A4 promoter via the pregnane X receptor (PXR). In mice, oral budesonide administration (25 mg/kg) for 4 days induced the murine homolog Cyp3a11 in the intestine 3-fold, whereas liver expression was notably less influenced. In knockout mice devoid of PXR, budesonide-mediated inductions were reduced compared to wild-type mice. In conclusion, we could demonstrate that budesonide is not an efficient inhibitor but rather an inducer of CYP3A via a PXR-mediated mechanism. In vivo, however, oral budesonide administration to mice showed only modest gene induction, which occurred mainly in the intestine. Therefore, the risk for budesonide-mediated drug interactions seems to be low but cannot be ruled out entirely.

Characterization of Gastrointestinal Drug Absorption in Cynomolgus Monkeys

Possible factors of species differences in gastrointestinal drug absorption between cynomolgus monkeys and humans were examined using several commercial drugs. Oral bioavailability (BA) of acetaminophen, furosemide, and propranolol in cynomolgus monkeys was significantly lower than that in humans. From the pharmacokinetic analysis, these drugs were found to show the low fraction absorbed into portal vein (FaFg), suggesting that the low BA in cynomolgus monkeys was attributed mainly to the gastrointestinal absorption processes. The gastric emptying rate (GER) calculated from plasma concentration profiles after oral administration of acetaminophen in cynomolgus monkeys was similar in humans. The gastrointestinal transit time (GITT) in cynomolgus monkeys was only slightly shorter than that in humans. On the other hand, it was demonstrated that the apparent intestinal permeability (Papp) of five drugs to cynomolgus monkey intestine was lower than that to rat intestine; especially propranolol and furosemide showed the remarkably low Papp. The expression levels of mRNAs of efflux transporters analyzed by real-time RT-PCR indicated that mRNA expression levels of MDR1, MRP2, and BCRP in monkey intestine were significantly higher than those in human intestine. This result suggested that low oral absorption of furosemide in cynomolgus monkeys was attributed to the high activities of efflux transporters in its intestinal membrane. Results of in vivo PK analysis clearly showed that FaFg values of propranolol and acetaminophen in cynomolgus monkeys were markedly lower than those in humans. Since propranolol and acetaminophen were the drug with high membrane permeability, it was considered that the high first-pass metabolism in the enterocytes was a main factor of their low FaFg in cynomolgus monkeys. In conclusion, it was demonstrated that the high activities of efflux transporters and/or metabolizing enzymes in the intestinal membrane are possible factors to cause poor oral absorption of drugs in cynomolgus monkeys.

Visualization of First-Pass Drug Metabolism of Terfenadine by MALDIImaging Mass Spectrometry

The aim of this article is to focus on the implementation and the application of matrix-assisted laser desorption/ionization-imaging mass spectrometric system (MALDI-IMS) to determine the disposition or biotransformation pathway of terfenadine and its active metabolite, fexofenadine in mouse and rat whole-body tissue sections. Whole-body MALDI-IMS data showed that the poor oral bioavailability of terfenadine was largely due to high first-pass metabolism in the intestines and the liver before the compound reached systemic circulation.

SEX DIFFERENCES IN THE PHARMACOKINETICS, OXIDATIVE METABOLISM AND ORAL BIOAVAILABILITY OF OXYCODONE IN THE SPRAGUE‐DAWLEY RAT

1. The pharmacokinetics and oxidative metabolism of oxycodone were investigated following intravenous and oral administration in male and female Sprague-Dawley (SD) rats. 2. High-performance liquid chromatography (HPLC)-electrospray ionization (ESI)-tandem mass spectrometry (MS-MS) was used to quantify plasma concentrations of oxycodone and its oxidative metabolites noroxycodone and oxymorphone following administration of single bolus intravenous (5 mg/kg) and oral (10 mg/kg) doses of oxycodone. 3. The mean (+/-SEM) clearance of intravenous oxycodone was significantly higher in male than female SD rats (4.9 +/- 0.3 vs 3.1 +/- 0.3 L/h per kg, respectively; P < 0.01). Mean areas under the plasma concentration versus time curves (AUC) for oxycodone were significantly higher in female than male SD rats following intravenous (approximately 1.6-fold; P < 0.01) and oral (approximately sevenfold; P < 0.005) administration. 4. The oral bioavailability of oxycodone was low (at 1.2 and 5.0%, respectively) in male and female SD rats, a finding consistent with high first-pass metabolism. Noroxycodone : oxycodone AUC ratios were significantly higher in male than female SD rats after intravenous (approximately 2.4-fold; P < 0.005) and oral (approximately 12-fold; P < 0.005) administration. 5. Circulating oxymorphone concentrations remained very low following both routes of administration. Noroxycodone : oxymorphone AUC ratios were greater in male than female SD rats after intravenous (approximately 13- and fivefold, respectively) and oral (approximately 90- and sixfold, respectively) administration. 6. Sex differences were apparent in the pharmacokinetics, oxidative metabolism and oral bioavailability of oxycodone. Systemic exposure to oxycodone was greater in female compared with male SD rats, whereas systemic exposure to metabolically derived noroxycodone was higher in male than female SD rats. 7. Oral administration of oxycodone to the SD rat is a poor model of the human for the study of the pharmacodynamic effects of oxycodone.

Splanchnic Oxidation Is the Major Metabolic Fate of Dietary Glutamate in Enterally Fed Preterm Infants

The intestine is a major site of amino acid metabolism, especially in neonates. The energy needed for the metabolic processes in neonatal animals is derived from dietary glucose and amino acids. No data are available showing that dietary amino acids function as intestinal fuel source in human neonates as well. We hypothesized that preterm infants show a high splanchnic first-pass glutamate metabolism and the primary metabolic fate of glutamate is oxidation. Five preterm infants (birth weight 1.2+/-0.2 kg, gestational age 29+/-1 wk) were studied by dual tracer ([U-(13)C]glutamate and [D3]glutamate) techniques on two study days (within postnatal d 14-19). Splanchnic and whole-body glutamate kinetics were assessed by plasma isotopic enrichment of [U-(13)C]glutamate and [D3]glutamate and breath CO2 enrichment. Fractional first-pass glutamate uptake was 77+/-18% on d 1, and 70+/-7% on d 2, mean 74+/-13%. Almost all (86+/-7%) of the glutamate used in the first pass is directed toward oxidation. There is a high splanchnic fractional first-pass uptake and a high oxidation rate of glutamate in preterm infants. Glutamate is an important source of energy for the splanchnic tissues in preterm infants receiving full enteral feeding.

Preclinical pharmacokinetics and in vitro metabolism of dasatinib (BMS-354825): a potent oral multi-targeted kinase inhibitor against SRC and BCR-ABL

Dasatinib (BMS-354825), a potent oral multi-targeted kinase inhibitor against SRC and BCR-ABL, has recently been approved for the treatment of chronic myelogenous leukaemia (CML) in imatinib-acquired resistance and intolerance. In vitro and in vivo studies were conducted to characterize the pharmacokinetics and metabolism of dasatinib in mouse, rat, dog, and monkey. Possible mechanisms contributing to the incomplete oral bioavailability of dasatinib in animals were investigated. Metabolic stability of dasatinib was measured after incubation with liver microsomes (either NADPH- or UDPGA-fortified) and isolated hepatocytes obtained from mouse, rat, dog, monkey, and human. In all cases, substrate depletion over time was measured, and appropriate scaling factors were used to predict in vivo clearance. Pharmacokinetics of dasatinib were determined in mice, rats, dogs, and monkeys after administration of single intravenous or oral doses. In addition, the routes of excretion were investigated after administration of dasatinib to bile duct cannulated (BDC) rats. Absorption and first-pass metabolism were evaluated as possible reasons for the incomplete oral bioavailability using various in vitro and in vivo models like Caco-2 cells, P-glycoprotein (P-gp) knockout mice, and intra-portal dosing in rats. In vivo systemic plasma clearance values of dasatinib were 62, 26, 25, and 34 ml/min/kg in mouse, rat, dog, and monkey, respectively. Scaling of in vitro hepatocyte and liver microsomal data gave reasonably good predictions of in vivo clearances across all species. Percent distribution in blood cells ranged from 43% in mouse to 57% in dog. Dasatinib showed high volumes of distribution (>3 l/kg) and high serum protein binding values (>90%) in all four species tested. Oral bioavailability of dasatinib ranged from 14% in the mouse to 34% in the dog. In rats, bioavailability after an intraportal dose was comparable to that after intra-arterial administration. In BDC rats, less than 15% of an intravenous dose was excreted unchanged in urine, bile, and the gastrointestinal tract, suggesting that dasatinib is cleared primarily via metabolism. Dasatinib has high intrinsic permeability in Caco-2 cells, however, the efflux ratio was approximately two-fold indicating that it may be a substrate for an intestinal efflux transporter. However, in vivo studies in P-gp knockout mice versus wild-type mice showed no difference in the amount of dasatinib remaining unabsorbed in the gastrointestinal tract, suggesting that P-gp may not be responsible for the incomplete bioavailability. Dasatinib shows intermediate clearance in mouse, rat, dog, and monkey, and distributes extensively in those species. Oxidative metabolism appears to be the predominant clearance pathway. The incomplete oral bioavailability may be due to both incomplete absorption and high first-pass metabolism. However, the efflux transporter, P-glycoprotein does not appear to be limiting oral absorption.

Polarized provision of cysteine affects redox homeostasis in intestinal Caco2‐BBE cells

Prior evidence for high first-pass metabolism of enteral cysteine led to the hypothesis that the lack of luminal cysteine with parenteral feeding may perturb redox homeostasis in the intestinal epithelium. To explore possible underlying mechanisms, polarized Caco2-BBE intestinal epithelial cells were seeded on cell culture inserts, grown to confluence determined by trans-epithelial electronic resistance and subsequently provided cysteine in apical, basal, both, or neither compartments. HPLC analysis revealed a decrease (P < 0.05) in GSH concentrations in cells receiving cysteine from the basal compartment compared to apical only or both compartments determined 7d post-confluence (differentiated cells). Similarly, redox potential as determined by the Nernst equation was significantly greater (P < 0.05) with apical (−205 mV) versus basal (−192 mV) provision of cysteine. These data indicate that differentiated intestinal epithelial cells may rely on apical provision of cysteine for the maintenance of redox homeostasis. This possibility was examined further by measuring 35S-cysteine uptake and γ-glutamylcysteine synthetase (γ-GCS) RNA expression. Apical provision of cysteine spiked with 35S-cysteine increased (P < 0.05) cellular radioactivity compared to basal supply indicating greater absorption from the apical surface. Quantitative RT-PCR analysis demonstrated that steady-state γ-GCS RNA expression was also greater with apical versus basal cysteine provision. Together, these data indicate the importance of apical provision of cysteine for maintenance of redox homeostasis in Caco2-BBE cells. This work was supported by the NIH (R01 DK061568)

Mutual prodrug concept: Fundamentals and applications

A therapeutically significant drug may have limited utilization in clinical practice because of poor organoleptic properties, poor bioavailability, short duration of action, nonspecificity, incomplete absorption, poor aqueous solubility, high first-pass metabolism or other adverse effects. There is a great emphasis on research to discover methods aimed at improving their therapeutic efficacy by minimizing or eliminating these undesirable properties. Sometimes, an adequate pharmaceutical formulation can overcome these drawbacks, but often the galenic formulation is inoperant and a chemical modification of active molecule is necessary to correct its pharmacokinetic insufficiencies. This chemical formulation process, whose objective is to convert an interesting active molecule into a clinically acceptable drug, often involves the so-called ‘Prodrug design.’ Mutual prodrug is a type of carrier-linked prodrug, where the carrier used is another biologically active drug instead of some inert molecule. A mutual prodrug consists of two pharmacologically active agents coupled together so that each acts as a promoiety for the other agent and vice versa. Mutual prodrug design is really no different from the general drug discovery process, in which a unique substance is observed to have desirable pharmacological effects, and studies of its properties lead to the design of better drugs. It is a very fruitful area of research, and its introduction in human therapy has given successful results in improving the clinical and therapeutic effectiveness of drugs suffering from some undesirable properties that otherwise hinder their clinical usefulness. The present article takes a review of various applications of mutual prodrugs and the developments in this field during the last few decades.

Effect of Excipient and Processing Variables on Adhesive Properties and Release Profile of Pentoxifylline From Mucoadhesive Tablets

ABSTRACTThe bioavailability and onset of action of drugs with high first-pass metabolism can be significantly improved by administration via the sublingual route. The objective of this study was to evaluate the effect of polymer type and tablet compaction parameters on the adhesive properties and drug release profile from mucoadhesive sublingual tablet formulations. Pentoxifylline was selected as the model drug because it has poor oral bioavailability due to extensive first-pass metabolism. Two polymers known to possess mucoadhesive properties, carbomer and hydroxypropyl methyl cellulose (HPMC), were used to prepare the formulations. Tablets were prepared by using direct compression technique and evaluated for in vitro dissolution, drug-excipient interactions, and adhesive properties. In general, there was a decrease in the rate of drug release with an increase in the concentration of polymers. No drug-excipient interactions were evident from differential scanning calorimetry or high-performance liquid chromatography analysis. For the formulations containing HPMC, the force of mucoadhesion increased with an increase in the concentration of polymer; however, for carbomer formulations, no such correlation was observed. Force of mucoadhesion decreased as a function of hydration time in both of the polymers.

Bioavailability and Metabolism of Prochlorperazine Administered via the Buccal and Oral Delivery Route

Prochlorperazine has been accepted as an effective antiemetic for more than 50 years; however, its therapeutic success has been limited by its low and variable absorption and high first-pass metabolism. A buccal dosage form of prochlorperazine has been developed. This article discusses 2 clinical studies conducted to characterize the single-dose and multiple-dose pharmacokinetics of prochlorperazine and its metabolites after buccal administration. The results of these studies demonstrate that buccal administration of prochlorperazine produces plasma concentrations more than twice as high as an oral tablet, with less than half the variability. In addition to the metabolites, N-desmethyl prochlorperazine and prochlorperazine sulfoxide, 2 new metabolites, prochlorperazine 7-hydroxide and prochlorperazine sulfoxide 4'-N-oxide, were identified and quantitated. Exposure to metabolites after the buccal prochlorperazine formulation was approximately half that observed after the oral tablet. Buccal administration of prochlorperazine, twice daily, should enhance the therapeutic role of prochlorperazine in preventing and treating nausea and vomiting.

Randomized, Double-Blind, Placebo-Controlled Study About the Effects of Cannabidiol (CBD) on the Pharmacokinetics of ??9-Tetrahydrocannabinol (THC) After Oral Application of THC Verses Standardized Cannabis Extract

Cannabidiol (CBD) is known to modify the effects of Delta-tetrahydrocannabinol (THC) by decreasing anxiety and antagonizing other THC-effects. As a reason, pharmacodynamic as well as pharmacokinetic mechanisms were suggested. In context of the use of cannabis-based medicine extracts for therapeutic purposes, a study was performed in a double-blind and placebo-controlled cross-over design in which each of 24 volunteers (12 male and 12 female, age 18-45 years) obtained soft-gelatin capsules with 10 mg THC (THC-set), cannabis extract containing 10 mg THC +5.4 mg CBD (CAN-set) or placebo in weekly intervals. Blood samples were taken 30 minutes before and 30 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 9 hours and 24 hours after the intake. The concentrations of THC, of its metabolites 11-OH-THC, THC-COOH and of CBD in the plasma samples were determined by automatic solid phase extraction, derivatization with N,O-bis(trimethylsilyl)triflouroacetamide and gas chromatography-mass spectrometry. The concentration versus time curves (maximum concentrations Cmax, corresponding time tmax and areas under the curves AUC) were evaluated by statistical methods with respect to equivalence or differences between the CAN-set and the THC-set. Furthermore, the intra-individual ratios of Cmax and AUC for 11-OH-THC/THC, THC-COOH/THC and THC-COOH/11-OH-THC were compared between the THC-set and the CAN-set. Despite the large variation of the data, evidence emerged from the total of the results that CBD partially inhibits the CYP 2C catalyzed hydroxylation of THC to 11-OH-THC. The probability for this inhibition is particularly high for oral intake because THC and CBD attain relatively high concentrations in the liver and because of the high first-pass metabolism of THC. However, the effect of CBD is small in comparison to the variability caused by other factors. Therefore, a pharmacokinetic reason for the differences determined between pure THC and cannabis extract is improbable at the doses chosen in this study. Significantly higher AUC and Cmax and shorter tmax were found for females as compared with males.

Bioavailability and Pharmacokinetic Studies of Eurycomanone fromEurycoma longifolia

A validated HPLC analysis of eurycomanone (1), a bioactive quassinoid, in rat plasma following oral and intravenous administration of Eurycoma longifolia Jack extract was developed for pharmacokinetic and bioavailability studies. Relatively high plasma eurycomanone concentrations were detected after an intravenous injection of 10 mg/kg extract F2 containing 1.96 mg/kg of the quassinoid. However, it declined rapidly to zero after 8 h. Its mean elimination rate constant (k(e)), biological half-life (t(1/2)), volume of distribution (V(d)) and clearance (CL) were 0.88 +/- 0.19 h (-1), 1.00 +/- 0.26 h, 0.68 +/- 0.30 L/kg and 0.39 +/- 0.08 L/h/kg, respectively. Following oral administration of eurycomanone, its Cmax and Tmax values were detected as 0.33 +/- 0.03 microg/mL and 4.40 +/- 0.98 h, respectively. The plasma concentration of the quassinoid after oral administration was much lower than after intravenous application in spite of the oral dose being 5 times higher. The results indicate that eurycomanone is poorly bioavailable when given orally. A comparison of the AUC (0-->infinity) obtained orally to that obtained after an intravenous administration (normalized for dose differences) revealed that the absolute bioavailability of the compound was low with 10.5 %. Furthermore, the compound appeared to be well distributed in the extravascular fluids because of its relatively high V(d) value. The poor oral bioavailability was not attributed to instability problems because eurycomanone has been shown to be stable under different pH conditions. Thus, its poor oral bioavailability may be due to poor membrane permeability in view of its low P value and/or high first-pass metabolism.

Budesonide combined with UDCA to improve liver histology in primary biliary cirrhosis

Ursodeoxycholic acid (UDCA) is a safe medical therapy for primary biliary cirrhosis (PBC), but its effect on liver histology remains uncertain. Budesonide is a glucocorticoid with high receptor activity and high first-pass metabolism in liver. We evaluated the combination of budesonide and UDCA on liver histology and compared this with UDCA alone in a 3-year prospective, randomized, open multicenter study. Patients with PBC (n = 77), at stages I to III, were randomized into 2 treatment arms, A (n = 41): budesonide 6 mg/d and UDCA 15 mg/kg/d and B (n = 36): UDCA 15 mg/kg/d. Liver histology was assessed at the beginning and at the end of the study. Liver function tests and glucose and cortisol values were determined every 4 months. Paired liver biopsy specimens were available from 69 patients (A = 37 and B = 32). Stage improved 22% in group A but deteriorated 20% in group B (P = .009). Fibrosis decreased 25% in group A but increased 70% in group B (P = .0009). S-PIIINP decreased significantly in group A. Inflammation decreased in both groups, 34% in group A (P = .02), but only 10% in group B (P = NS). Serum liver enzymes decreased significantly in both treatment arms. Bilirubin values rose in group B but stayed stable in group A (A/B P = .002). A mild systemic glucocorticoid effect from budesonide was evident after 2 years. In conclusion, budesonide combined with UDCA improved liver histology, whereas the effect of UDCA alone was mainly on laboratory values. Studies with longer follow-up using a combination of budesonide and UDCA are warranted to confirm safety and effects.

Meta-analysis: budesonide treatment for collagenous colitis.

Collagenous colitis is typified by chronic watery diarrhoea and characteristic histological alterations of the colonic mucosa without endoscopic abnormalities. Budesonide, a corticosteroid with high first-pass metabolism has been examined in collagenous colitis, but studies to date have had small numbers, and relatively low statistical power. A meta-analysis of existing published trials was undertaken to evaluate the treatment effect of budesonide in collagenous colitis. All pertinent literature sources were searched for published reports in English of budesonide use in collagenous colitis. MEDLINE and EMBASE databases were reviewed, as well as bibliographies from published articles and available abstracts from relevant meetings. Literature that met prespecified criteria was selected for the meta-analysis. Three trials were included in the meta-analysis. Budesonide significantly decreased stool frequency (budesonide vs. placebo OR: 20.1, 95% CI: 7.0-57.5, P < 0.0001). In general, budesonide treatment was well-tolerated. Budesonide is clinically effective short-term in collagenous colitis, and seems to be relatively well-tolerated. Clinicians can consider this drug as a reasonable option for patients with this disorder.

SURVEY OF ADRENAL CRISIS ASSOCIATED WITH INHALED CORTICOSTEROIDS IN THE UNITED KINGDOM

Todd GR, Acerini CL, Ross-Russell R, et al. Arch Dis Child. 2002;87:457–461Rare reports of acute adrenal crisis associated with inhaled corticosteroid (ICS) use have been published. How commonly does this occur? To which patients? At what dose? With which drugs?Patients of pediatricians and endocrinologists in the United Kingdom were studied.Questionnaires were sent to the physicians, asking whether they had encountered asthmatic patients with acute adrenal crises associated with ICS use. Physicians who responded positively completed a more detailed questionnaire. Patients receiving orally administered corticosteroids were excluded, and the case definition required both symptoms of an adrenal crisis and abnormal hypothalamic-pituitary-adrenal axis function test results.Thirty-three patients met the case definition criteria for acute adrenal crises developing in relation to ICS therapy, including 28 children (mean age: 6.4 years; range: 3.3–10 years) and 5 adults. Twenty-three children presented with acute hypoglycemia (13 with decreased levels of consciousness or coma, 9 with coma and convulsions, and 1 with coma, convulsions, and death). The remainder of the children and the majority of the adults presented with a more insidious onset of symptoms, such as lassitude, weakness, nausea, and dizziness. There were 37 total episodes of adrenal crisis among the 33 patients. There was no obvious precipitating cause in 24 cases (65%), there was evidence of infection (mostly respiratory) in 8 cases (21%), the ICS had been stopped, reduced, or changed to a lower-potency ICS in 4 cases (11%), and 1 episode (3%) occurred postoperatively. The vast majority of child and adult patients (30 of 33 patients) were treated with fluticasone; 1 child was treated with both fluticasone and budesonide, and 1 adult and 1 child were treated with beclomethasone. The mean dose of fluticasone among children was 980 μg/day (range: 500–2000 μg/day), and the mean dose among adults was 1380 μg/day (range: 1000–2000 μg/day). The mean durations of ICS treatment were 1.7 years for children and 3.3 years for adults.The frequency of acute adrenal crises was greater than expected. Despite being the least prescribed and most recently introduced ICS, fluticasone was associated with 94% of the cases.Clearly ICSs can cause adrenal suppression and consequent adrenal crises. This appears to be especially true with fluticasone. The author of an editorial that accompanied this article explained that, although fluticasone has high first-pass hepatic metabolism, which decreases the systemic bioavailability of the swallowed portion of the dose, it also has high lipophilicity, allowing the pulmonary portion of the dose to be easily absorbed, which, “combined with its high receptor affinity and prolonged duration of activity, ensure systemic potency and accumulation.” Once again, we are reminded to use the lowest effective dose of ICS. Furthermore, when higher doses are required, perhaps an ICS other than fluticasone would be a better choice.

Chronic diarrhea

Chronic diarrhea

Subcutaneously administered apomorphine: pharmacokinetics and metabolism.

Apomorphine is a non-narcotic morphine derivative that acts as a potent dopaminergic agonist. Its high first-pass hepatic metabolism prevents effectiveness by the oral route; instead, subcutaneous injection is the usual route, and intranasal, sublingual, rectal, and iontophoretic transdermal delivery has been investigated for the treatment of Parkinson's disease (PD). The rate of uptake after subcutaneous injection is influenced by factors such as location, temperature, depth of injection, and body fat. Studies have shown the latency of onset to clinical effect after s.c. injection ranged from 7.3 to 14 minutes. Cerebrospinal fluid T(max)lags behind plasma T(max) by 10 to 20 minutes. Considerable intersubject variability is found with pharmacokinetic variables; in some studies there are five- to tenfold differences in C(max)and area-under-the-concentration-time-curve seen in PD patients. Apomorphine metabolism occurs through several enzymatic pathways, including N-demethylation, sulfation, glucuronidation, and catechol-O-methyltransferase as well as by nonenzymatic oxidation. The complexities of apomorphine uptake, distribution, and clearance probably contribute to its variability of clinical actions.

Lopinavir/ritonavir: a review of its use in the management of HIV infection.

Lopinavir/ritonavir: a review of its use in the management of HIV infection.

A reply:

In their letter to the editor, Josefsson & Ahlner correctly admit that grapefruit juice has no appreciable effect on the pharmacokinetics and pharmacodynamics of amlodipine. However, they argue that in their paper [1], Cmax as well as AUC were significantly (P < 0.05) higher when 5 mg amlodipine was administered with 250 ml of grapefruit juice compared with when administered with water. They claim that variability in the pharmacokinetics of amlodipine rather than variability due to analytical methods underlie the differences in the conclusions from their paper compared with the results from our study [2], although they did not indicate the confidence intervals of the ratios of their Cmax and AUC. In addition, Josefsson & Ahlner claim that whereas they showed data from all their subjects, we excluded one half of our subjects and implicitly skewed our results. While pharmacokinetic variations due to biological factors are feasible, the magnitude of the mean absolute bioavailability for amlodipine reported in our study is in agreement with data reported independently by Abernethy et al.[3], as well as data on file from a study in mildly hypertensive patients. With an absolute bioavailability > 80%, bioavailability enhancement as described for calcium channel blockers with a high first-pass metabolism when grapefruit juice is concurrently administered is unlikely to be significant with amlodipine, even if one assumes complete absorption. There is a correlation between blood pressure fall and plasma concentrations of calcium channel blockers [4–6], including amlodipine [3]. In studies with felodipine and nisoldipine, significant changes in exposure following concurrent administration of grapefruit juice were accompanied by changes in haemodynamics [7, 8]. The absence of any change in blood pressure or heart rate in their study is therefore consistent with the absence of any meaningful changes in blood concentrations of amlodipine. The assertion that we excluded subjects in our analysis is a complete misrepresentation of our paper. Pharmacokinetic and pharmacodynamic data from all 20 subjects were used in the analyses of both the oral and intravenous arms of the data on unresolved amlodipine. Plasma samples collected during our study were assayed for unresolved amlodipine prior to enantiomer assays being run. In a few subjects, the volume of plasma was inadequate at all time points for the enantiomer assays. We correctly identified all such data in our manuscript. We believe that the conclusions from our study have not been compromised because of the missing enantiomer data, recognizing the consistency of the data shown. There were also substantial differences between the conditions and design of our study and the study reported by Josefsson & Ahlner. We used a larger sample size of 20 subjects compared with their 12 subjects. We also used a higher dose of amlodipine (10 vs 5 mg), to ensure a more appropriate characterization of plasma concentrations over a longer interval. In our study, grapefruit juice was administered with amlodipine on the day of dosing and repeated daily with breakfast for the 9 days during which blood samples were obtained for pharmacokinetics. This profile more clearly mimics real life situations for subjects who routinely drink grapefruit juice with breakfast when medications are commonly consumed. In addition, the conditions of our study would more readily demonstrate any potential interaction, recognizing that grapefruit juice was on board continuously. Finally, we studied both oral and intravenous amlodipine to more appropriately define its pharmacokinetics under the conditions of our study. The results with both formulations were consistent for both unresolved amlodipine and the enantiomers. In conclusion, while it may be argued that normal biological variability could influence the pharmacokinetic data shown in these studies, it is unfortunate that Josefsson & Ahlner misrepresented our data in their letter. We agree with the conclusion in their paper that grapefruit juice does not alter the pharmacokinetics and pharmacodynamics of amlodipine.

Transdermal Delivery of Nicardipine: An Approach to In Vitro Permeation Enhancement

Nicardipine hydrochloride (NC-HCl), a calcium channel blocker for the treatment of chronic stable angina and hypertension, seems to be a potential therapeutic transdermal system candidate, mainly due to its low dose, short half-life, and high first-pass metabolism. The objective of the present study was to evaluate its flux and elucidate mechanistic effects of formulation components on transdermal permeation of the drug through the skin. Solubility of NC-HCl in different solvent systems was determined using a validated HPLC method. The solubility of drug in various solvent systems was found to be in decreasing order as propylene glycol (PG)/oleic acid (OA)/dimethyl isosorbide (DMI) (80:10:10 v/v) > PG > PG/OA (90:10 v/v) > polyethylene glycol 300 > ethanol/PG (70:30 w/w) > transcutol > dimethyl isosorbide (DMI) > ethanol > water and buffer 4.7 > 2-propanol. Propylene glycol was then selected as the main vehicle in the development of a transdermal product. As a preliminary step to develop a transdermal delivery system, vehicle effect on the percutaneous absorption of NC-HCl was determined using the excised skin of a hairless guinea pig. Vehicles investigated included pure solvents alone and their selected blends, chosen based on the solubility results. In vitro permeation data were collected at 37°C, using Franz diffusion cells. The skin permeation was then evaluated by measuring the steady state permeation rate (flux) of NC-HCl, lag time, and the permeability constant. The results showed that no individual solvent was capable of promoting NC-HCl penetration. Permeation profiles of the drug through hairless guinea pig skin using saturated solutions of drug were constructed. Among the systems studied, the ternary mixture of PG/OA/DMI and binary mixture of PG/OA showed excellent flux. The flux value of the ternary system was nearly three times higher than the corresponding values obtained for the binary solvent. A similar trend also was observed for the permeation conture stant, while the values of lag time were reversed. The ternary mixindicated was then selected as a potential absorption enhancement vehicle for the transdermal delivery of drug. In general, higher fluxes were observed through hairless guinea pig skin as compared with the human stratum corneum. Based on the results obtained from the release study of NC-HCl from saturated solutions of the drug, a novel lecithin organogel (microemulsion-based gel) composed of soybean lecithin, propylene glycol, oleic acid, dimethyl isosorbide, and isopropyl myristate was developed as a possible matrix for transdermal delivery of NC-HCl. In vitro percutaneous penetration studies from this newly developed gel system through giunea pig skin and human stratum corneum revealed that the organogel system has skin-enhancing potential and could be a promising matrix for the transdermal delivery of nicardipine. Furthermore, higher permeation rates were observed when nicardipine free base was incorporated into the gel matrix instead of hydrochloride salt.