Drug Disposition Studies Research Articles

Synthesis and analysis of the opioid analgesic [14C]-fentanyl

The synthesis of [14C]-fentanyl, the radiolabelled congener of the potent opioid analgesic chosen for utilization in drug disposition studies, is described. [14C]-Labelling was achieved in the first of two steps, a room temperature reduction of the in situ generated Schiff base from 1-phenylethyl-4-piperidone and [UL-14C]-aniline hydrochloride with sodium triacetoxyborohydride. A nearly instantaneous production of fentanyl was accomplished at room temperature with the addition of propionyl chloride. The overall radiochemical yield was 18%. The method described is efficiently adaptable for submicromolar scale while yielding a product of sufficient specific activity for in vivo studies. Our solvent system for thin layer chromatography was superior to the USP system reported for chromatographic analysis of fentanyl. This is the first reported preparation of [14C]-fentanyl with the radiolabel in the aniline benzene ring.

Elucidating the relationship between acetazolamide plasma protein binding and renal clearance using an albumin infusion.

The effect of plasma protein binding changes on drug clearance is an important concept in clinical pharmacology. In a hypoalbuminemic patient receiving acetazolamide, albumin infusion (50 g) increased acetazolamide plasma protein binding towards normal as the serum albumin concentration rose (r = 0.91, P < .001). The ratio of acetazolamide renal plasma clearance to creatinine clearance decreased as serum albumin levels increased (r = 0.78, P < .05) and the unbound drug fraction fell (r = 0.88, P < .01), but clearance ratios based on unbound plasma acetazolamide levels did not change. Albumin infusion resulted in a nonparallel decline over time between plasma and unbound plasma acetazolamide concentrations. These data demonstrate that, over the range of observed serum albumin concentrations, acetazolamide renal plasma clearance is sensitive to changes in plasma protein binding. Furthermore, our findings emphasize the importance of measuring unbound drug levels when protein binding changes occur during the course of drug disposition studies. Finally, this methodology allows for the fascile assessment of the effects of plasma protein binding changes on renal drug clearance.

Synthesis of (R)‐5‐(Di[2,3‐3H2]propylamino)5,6‐dihydro‐4H‐imidazo[4,5,1‐ij]quinolin‐2(1H)‐one ([3H]U‐86170) and (R)‐5‐([2,3‐3H2]Propylamino)‐5,6‐dihydro‐4H‐imidazo[4,5,1‐ij]quinolin‐2(1H)‐one ([3H]U‐91356)

Abstract(R)‐5‐(diallylamino)‐5,6‐dihydro‐4H‐imidazo[4,5,1‐ij]quinolin‐2(1H)‐one (12b) was prepared in 9% overall yield from 3‐aminoquinoline. Reaction of 12b in ethyl acetate with tritium gas in presence of a 5% platinum on carbon catalyst afforded a mixture of (R)‐5‐(di[2,3‐3H2]propylamino)‐5,6‐dihydro‐4H‐imidazo[4,5,1‐ij]‐quinolin‐2(1H)‐one ([3H]U‐86170, 13, 69 Ci/mmol) and (R)‐5‐([2,3‐3H2]‐propyl‐amino)‐5,6‐dihydro‐4H‐imidazo‐[4,5,1‐ij]quinolin‐2(1H)‐one ([3H]U‐91356, 14, 34 Ci/mmol) which was separated by preparative reverse‐phase chromatography. U‐86170 and U‐91356 are potent dopamine D2 agonists. The labelled compounds are useful for drug disposition studies. [3H]U‐86170 is also useful as a dopamine D2 agonist radioligand for receptor binding studies.

Synthesis of [119mSn]‐mesoporphyrin IX dichloride

AbstractTin mesoporphyrin IX dichloride (Sn‐MPCl2) is a heme oxygenase inhibitor of current clinical interest for the treatment of neonatal hyperbilirubinemia. The synthesis of [119mSn]‐MPCl2 for drug metabolism and disposition studies is reported. [119mSn]‐MPCl2 was prepared in 60% radiochemical yield by metalation of the porphyrin nucleus of mesoporphyrin IX dihydrochloride with tin(II)‐119m acetate. The product had a specific activity of 43.4 mCi/mmol and a radiochemical purity of 99%, as determined by radio‐HPLC analysis.

Peptide transport across the blood-brain barrier

The progress in isolation and culturing techniques of cerebrovascular endothelial cells, which will be briefly reviewed, enabled the development of in vitro models of the blood-brain barrier. The in vitro models, which are by now extensively characterized and validated, can generally be applied to specific aspects of blood-brain barrier anatomy and physiology and can also offer a valuable tool in drug disposition studies. Application of the in vitro model to the specific question of peptide transport across the blood-brain barrier, offers numerous methodological advantages: basic physiological parameters can be controlled and different experimental setups, not available in the in vivo setup, can be applied, thereby offering means to obtain a more thorough understanding of the basic aspects and mechanisms responsible for peptide transport across the blood-brain barrier. This approach will be illustrated by means of studies performed on a vasopressin-like model-peptide and will be presented in perspective of in vivo studies performed with the same peptide. Potential application of the in vitro blood-brain barrier model also includes the field of transport enhancement of poorly transported drugs, a maybe essential prerequisite for further development of peptides into therapeutically beneficial drugs.

Synthesis of tritium labelled 1‐(3,4‐dichlorophenyl)‐3‐(methylamino)propanol hydrochloride

Abstract1‐(3,4‐Dichlorophenyl)‐3‐(methylamino)propanol hydrochloride, 1, a potential antidepressant, was synthesized by a two‐step method in the [3H]‐labelled form with specific activity 12.5 mCi/mmol suitable for drug metabolism and disposition studies (1).

Synthesis of carbon‐14 and tritium labeled methylprednisolone suleptanate

AbstractMethylprednisolone suleptanate was initially labeled with tritium in the A‐ring of the steroid portion of the molecule, and with carbon‐14 at both carboxylic carbons of the suberic acid portion of the side chain. However these labels proved to lack total metabolic stability after administration to rats. Subsequently a second pair of labeled methylprednisolone suleptanates was synthesized, with tritium at C‐7 in the B‐ring of the steroid and carbon‐14 exclusively at the carboxamide carbon in the side chain. These labeled compounds showed excellent metabolic stability of both the tritium and carbon‐14 labels, and should be well suited for conducting drug disposition studies.

NMR studies of drug metabolism and disposition

The ways in which NMR is being used in vivo to study drug metabolism and disposition are reviewed. We also assess the role of this technique as a non-invasive method for monitoring the fate of drugs in human and animals, and for providing information about pharmacology and toxicity.

Reversed‐Phase LC resolutions of chiral antiarrhythmic agents via derivatization with homochiral isothiocyanates

The search for new antiarrhythmic agents has been intense, because the established drugs for the treatment of cardiac arrhythmias are neither uniformly effective nor well-tolerated. Among the recently introduced new antiarrhythmic agents are tocainide (TOC), mexiletine (MEX), flecainide (FLE), and propafenone (PRO). Each of these drugs is a chiral amine used clinically as the racemic mixture. We have examined the high-performance liquid chromatographic chiral resolution of the above four drugs via derivatization with homochiral derivatizing agents (HDAs). The amino functionality of the drugs was reacted with four homochiral isothiocyanates, 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate (TAGIT), (R)-alpha-methylbenzyl isothiocyanate (RAMBI), (S)-1-(1-naphthyl)ethyl isothiocyanate (SNEIT), and (R)-1-(2-naphthyl)ethyl isothiocyanate (RBEIT). Complete separation of the two peaks (resolution factor R = 1.5) was achieved with all four HDAs for TOC, with TAGIT, RBEIT, and RAMBI for MEX, with TAGIT and SNEIT for PRO, and only with TAGIT for FLE. SNEIT was used to develop analytical procedures for the determination of the enantiomeric composition of TOC in human urine and blood serum. The four HDAs offer several advantages over many other HDAs and should be useful in studies of enantioselective drug action and disposition.

Fungal metabolism of 4-substituted amphetamines

1. rac.-4-Ethoxyamphetamine was incubated with 14 different yeast and fungal microorganisms. 4-Hydroxyamphetamine was the major metabolite; traces of N-acetyl-4-ethoxyamphetamine were also detected. 2. The major fungal (Cunninghamella) metabolite of 4-propoxyamphetamine and 4-benzyloxyamphetamine was 4-hydroxyamphetamine. The major metabolites of 4-methoxyamphetamine were N-acetyl-4-methoxyamphetamine and 4-hydroxyamphetamine. 3. Acetoin derivatives were formed when alkoxyamphetamine substrates were incubated in the presence of various fungi and yeasts. 4. The findings indicate that Cunninghamella echinulata may be a useful microbial model for drug disposition and interaction studies.

Liposomal ophthalmic drug delivery. III. Pharmacodynamic and biodisposition studies of atropine

The potential of liposomes as an ophthalmic drug delivery system was investigated by comparing disposition and pupillary dilatory effect of atropine base and atropine sulphate in solution and various liposomal forms when topically instilled to the rabbit eye. Atropine base entrapped in multilamellar lipid vesicles (MLVs) with positive surface charge displayed the most prolonged effect lasting up to 12 h. MLVs with neutral and negative charges maintained the effect for 9 h while atropine in solution form was effective for only 7 h. Preparations containing atropine sulphate displayed a similar pattern although were shorter-acting than corresponding base products. All preparations whether salt or base, were equally effective in producing maximal response. In vivo drug disposition studies indicated the liposomal form produced significantly higher drug levels in the anterior tissues of the eye up to 8 h after instillation. Increased ocular bioavailability of atropine to these tissues was attributed to enhancement of pulse entry with little evidence of sustained drug release. It could be concluded that liposome encapsulation extended the duration of action and favourably altered disposition of atropine when topically instilled to the rabbit eye.

Anaesthetic influences on brain haemodynamics in the rat and their significance to biochemical, neuropharmacological and drug disposition studies

The aim of the present investigation was to examine, in the acute surgically-prepared rat, the effects that a range of commonly employed, parenterally administered general anaesthetic regimens may have upon brain perfusion

Pharmacokinetics of capacity-limited systems.

Nonlinearities are commonplace in pharmacokinetics and most frequently occur because of a limited concentration of biological material available for interaction with relatively high concentrations of drugs. The Michaelis-Menten-type function can be applied to metabolism, transport, and binding phenomena that display capacity-limitation. Nonlinear systems can be handled with differential equations (directly or converted to linear form), integrated equations (in non-closed form) or with relatively new area/moment or MRT relationships. Drug disposition studies should include several dose levels and recommendations are provided for detecting the presence of one or more nonlinear absorption or disposition processes in typical experimental data.

Propranolol metabolism by Cunninghamella bainieri.

1. Incubations of racemic propranolol alone or in the presence of either quinidine or sparteine were performed with Cunninghamella bainieri. 2. Five mammalian metabolites of propranolol (4-hydroxypropranolol, desisopropyl-propranolol, 1-naphthoxylactic acid, propranolol glycol and 1-naphthoxyacetic acid) were present in unhydrolysed extracts of the incubation medium according to h.p.l.c. and g.l.c. analyses. The relative proportion of 4-hydroxypropranolol increased after enzymic treatment. 3. Propranolol not only had a fungistatic effect, but also caused morphological changes in the organism, which were accompanied by decomposition of 4-hydroxypropranolol and formation of a greenish-brown colour in the incubation medium. 4. Drug interaction experiments yielded results which paralleled those reported in mammals. 5. The findings indicate that C. bainieri may be a useful microbial model for drug disposition and interaction studies.

Hydronephrosis in rats associated with postnatal exposure to a fatty acid oxidation inhibitor

Methyl palmoxirate, an inhibitor of long-chain fatty acid oxidation, was administered by gavage (0, 1, 5, or 20 mg/kg/day) to female rats over the last third of gestation and throughout lactation. Weight gain (mid- and high-dosage group) and survivability (high-dosage group) were significantly ( p ≤ 0.05) reduced in offspring of methyl palmoxiratetreated dams as compared to control offspring. Mid- and high-dosage male offspring found dead after Day 4 of lactation exhibited grossly distended bladders and renal pelves. A dosage-related increased incidence of dilated renal pelves was observed in both sexes at necropsy of 21-day-old mid- and high-dosage group pups. Microscopic examination of the urinary tracts of a number of affected pups revealed renal parenchymal atrophy and urethral obstruction. Drug disposition studies indicated lactating pups were exposed to significant amounts of methyl palmoxirate via mammary secretions. Cross-fostering experimentation suggested that some of the adverse effects observed in offspring were due to lactational, rather than in utero, exposure.

Pharmacokinetics of aminoglycoside antibiotics in blood, inner-ear fluids and tissues and their relationship to ototoxicity.

This review critically evaluates the literature on aminoglycoside pharmacokinetics in order to answer the question how fluid and tissue levels of the drugs relate to the development of ototoxic and nephrotoxic side effects. We will summarize the evidence that: (1) aminoglycosides do not accumulate in inner-ear fluids; (2) aminoglycoside levels in fluids do not correlate with the ototoxic potential of a drug, and (3) selective toxicity cannot be explained by selective tissue penetration of the drugs. We suggest that studies of drug disposition at the cellular level after chronic aminoglycoside treatment be conducted to establish whether a cell-specific uptake contributes to the selective toxicity of the aminoglycoside antibiotics. A sequence of biochemical events that may lead to the development of toxicity at the molecular level is briefly described.

Use of stable isotopes in drug disposition studies in man

Use of stable isotopes in drug disposition studies in man

The influence of nutrition on the systemic availability of drugs. Part II: Drug metabolism and renal excretion.

Plasma concentrations of an active compound vary between individuals and within a subject, even if the same drug dosage is used. These differences are related to variations of drug absorption from the gastrointestinal tract, of presystemic drug metabolism in the intestine or the liver, and of drug elimination from the systemic circulation, for instance by hepatic metabolism or renal excretion. All of these processes can be modified by nutrition. However it is necessary to emphasize the significance of the pharmaceutical formulation for variations of both drug absorption and elimination. The role of nutrition should receive adequate attention during therapeutic use of drugs as well as in drug disposition studies. Recent guidelines for the assessment of the systemic availability of drugs therefore do recommend nutrition to be controlled.

Discovery and anticonvulsant activity of the potent metabolic inhibitor 4-amino-N-(2,6-dimethylphenyl)-3,5-dimethylbenzamide.

Compound 2 [4-amino-N-(2,6-dimethylphenyl)benzamide] is an effective anticonvulsant in several animal models. For example, following oral administration to mice, it antagonized maximal electroshock (MES) induced seizures with an ED50 of 1.7 mg/kg. During drug disposition studies with 2, we found that it was rapidly metabolized by N-acetylation. Thirty minutes after oral administration of 1.7 mg/kg of 2 to mice, plasma concentrations of parent drug and the N-acetyl metabolite 5 were 1.09 and 0.41 microgram/mL, respectively. Six hours postadministration the concentrations were 0.23 and 0.22 microgram/mL, respectively. In order to sterically preclude or diminish the rate of metabolic N-acetylation, we synthesized analogues of 2 possessing either one (3) or two (4) methyl groups ortho to the 4-amino substituent. Both compounds antagonized MES-induced seizures after administration to mice; oral ED50 values for 3 and 4 were 3.5 and 5.6 mg/kg, respectively. Compound 3 was rapidly metabolized by N-acetylation. However, 4 provided exceptionally high and long-lived plasma concentrations of parent drug; no N-acetyl metabolite could be detected. While 2 and 3 had no pharmacologically relevant effects on hexobarbital-induced sleeping time in mice, 4 was a potent, dose-dependent potentiator of sleeping time. Oral administration of 375 micrograms/kg led to a 61% increase in sleeping time relative to control values. Thus, 4 represents one of the most potent potentiators of hexobarbital-induced sleeping time described to date.

Automated sample preparation and chromatographic analysis: determination of CGS 10787B and related compounds.

An automated method is described for analyzing biological samples originating from CGS 10787B drug disposition studies. The method incorporates a laboratory robot to prepare plasma and urine samples and a high performance liquid chromatographic system to simultaneously analyze for CGS 10787B, as well as metabolites and drug-related compounds (CGS 12094, CGS 17000, and CGS 17001). The robot allquots the biological sample, adds an internal standard, and performs all the steps necessary for the liquid-liquid extraction and concentration of the drug and related components, while operating unattended around the clock. Recovery and reproducibility assessments indicate good accuracy and precision for the method. The limits of detection for the method are 0.2 microgram/mL in plasma and 0.5 microgram/mL in urine, for all components.