Peter Timmins

Clinical Pharmacokinetics of Metformin

Metformin is widely used for the treatment of type 2 diabetes mellitus. It is a biguanide developed from galegine, a guanidine derivative found in Galega officinalis (French lilac). Chemically, it is a hydrophilic base which exists at physiological pH as the cationic species (>99.9%). Consequently, its passive diffusion through cell membranes should be very limited. The mean ± SD fractional oral bioavailability (F) of metformin is 55 ± 16%. It is absorbed predominately from the small intestine.Metformin is excreted unchanged in urine. The elimination half-life (t1/2) of metformin during multiple dosages in patients with good renal function is approximately 5 hours. From published data on the pharmacokinetics of metformin, the population mean of its clearances were calculated. The population mean renal clearance (CLR) and apparent total clearance after oral administration (CL/F) of metformin were estimated to be 510 ± 130 mL/min and 1140 ± 330 mL/min, respectively, in healthy subjects and diabetic patients with good renal function. Over a range of renal function, the population mean values of CLR and CL/F of metformin are 4.3 ± 1.5 and 10.7 ± 3.5 times as great, respectively, as the clearance of creatinine (CLCR). AS the CLR and CL/F decrease approximately in proportion to CLCR, the dosage of metformin should be reduced in patients with renal impairment in proportion to the reduced CLCR.The oral absorption, hepatic uptake and renal excretion of metformin are mediated very largely by organic cation transporters (OCTs). An intron variant of OCT1 (single nucleotide polymorphism [SNP] rs622342) has been associated with a decreased effect on blood glucose in heterozygotes and a lack of effect of metformin on plasma glucose in homozygotes. An intron variant of multidrug and toxin extrusion transporter [MATE1] (G>A, SNP rs2289669) has also been associated with a small increase in antihyperglycaemic effect of metformin. Overall, the effect of structural variants of OCTs and other cation transporters on the pharmacokinetics of metformin appears small and the subsequent effects on clinical response are also limited. However, intersubject differences in the levels of expression of OCT1 and OCT3 in the liver are very large and may contribute more to the variations in the hepatic uptake and clinical effect of metformin.Lactic acidosis is the feared adverse effect of the biguanide drugs but its incidence is very low in patients treated with metformin. We suggest that the mean plasma concentrations of metformin over a dosage interval be maintained below 2.5 mg/L in order to minimize the development of this adverse effect.

Steady-State Pharmacokinetics of a Novel Extended-Release Metformin Formulation

AbstractBackground and objective: Metformin is an effective treatment for type 2 diabetes mellitus. The pharmacokinetic characteristics of the conventional immediate-release (IR) formulation of metformin (Glucophage®), however, necessitate two- or three-times-daily dosing. Development of a novel extended-release (XR) formulation of metformin (Glucophage® XR) using GelShield Diffusion System technology provides a once-daily dosing option. The objective of this study was to assess the steady-state pharmacokinetics of metformin XR tablets. Study design: This was an open-label, multiple-dose, five-regimen, twosequence clinical study lasting 5 weeks. Methods: Subjects were 16 healthy volunteers aged 18–40 years. Three 1-week regimens of metformin XR (500, 1000 and 1500mg once daily) were administered sequentially. Subjects were alternately given either metformin XR 2000mg once daily or metformin IR 1000mg twice daily during weeks 4 and 5. The pharmacokinetic properties of metformin XR were assessed on two separate days at steady state and compared with those of metformin IR. Results: Absorption of metformin XR was slower than that of metformin IR (time to maximum plasma concentration = 7 versus 3 hours). Maximum plasma concentrations (Cmax) following the administration of metformin XR 2000mg once daily was 36% higher than that following the evening dose of metformin IR 1000mg twice daily. The extent of absorption, determined by area under the plasma concentration-time curve (AUC), was equivalent for both formulations. The mean accumulation ratio of metformin XR was 1.0, indicating no accumulation with multiple-dose administration. Intrasubject variabilities in Cmax and AUC of metformin were comparable between metformin XR and metformin IR. This novel formulation of metformin XR was well tolerated at single doses up to 2000mg once daily for 7 days, and adverse events were similar to those reported with metformin IR. Conclusion: The pharmacokinetic parameters of metformin XR tablet using GelShield Diffusion System technology were similar to those of metformin IR. Metformin XR was well tolerated at single doses up to 2000mg once daily.

Amorphous Drug-PVP Dispersions: Application of Theoretical, Thermal and Spectroscopic Analytical Techniques to the Study of a Molecule With Intermolecular Bonds in Both the Crystalline and Pure Amorphous State☆

We report the case of BMS-488043-PVP solid dispersions which when analysed using modulated DSC showed compliance with the Gordon-Taylor model, confirming ideal mixing behaviour of the two components. The nature or presence of stabilising interactions between drug and PVP could not be confirmed using this technique. Use of FT-IR, Raman and solid-state NMR spectroscopy confirmed the presence of stabilising hydrogen bond interactions between the drug and PVP. Similar interactions are present as intermolecular bonds in the crystalline and pure amorphous drug system. The Gordon-Taylor equation, as it is not predictive of the presence of intermolecular bonds such as hydrogen bonding in an amorphous dispersion, may underestimate the likely physical stability of solid dispersions which are produced and stabilised by these interactions.

Mechanisms of drug release in citrate buffered HPMC matrices

Few studies report the effects of alkalizing buffers in HPMC matrices. These agents are incorporated to provide micro-environmental buffering, protection of acid-labile ingredients, or pH-independent release of weak acid drugs. In this study, the influence of sodium citrate on the release kinetics, gel layer formation, internal gel pH and drug release mechanism was investigated in HPMC 2910 and 2208 (Methocel E4M and K4M) matrices containing 10% felbinac 39% HPMC, dextrose and sodium citrate. Matrix dissolution at pH 1.2 and pH 7.5 resulted in complex release profiles. HPMC 2910 matrices exhibited biphasic release, with citrate increasing the immediate release phase (<60min) and reducing the extended release. HPMC 2208 matrices were accelerated, but without the loss of extended release characteristics. Studies of early gel layer formation suggested gel barrier disruption and enhanced liquid penetration. pH modification of the gel layer was transitory (<2h) and corresponded temporally with the immediate release phase. Results suggest that in HPMC 2910 matrices, high initial citrate concentrations within the gel layer suppress particle swelling, interfere with diffusion barrier integrity, but are lost rapidly whereupon drug solubility reduces and the diffusion barrier recovers. These Hofmeister or osmotic-mediated effects are better resisted by the less methoxylated HPMC 2208.

Optimization and characterization of a pH-independent extended-release hydrophilic matrix tablet.

A simple systematic optimization approach was applied to tailor the drug release profile from a hydrophilic matrix extended-release tablet. When the ratio of anionic and nonionic polymers was optimized, pH-independent in vitro release of the model drug verapamil hydrochloride was obtained. The mechanisms of drug release at the pH extremes were evaluated by graphical analysis of the dissolution data and direct examination of the tablets during dissolution. Graphical evaluation did not completely clarify the release control mechanisms involved. Direct examination of tablets during dissolution, with estimation of amounts of drug and excipients dissolved at different times, gave further insight into relative contribution of mechanisms at different pH values. The change from predominantly diffusional to predominantly erosional mechanisms as pH is increased provides for the pH-independent release observed. This understanding should help model the application of this approach to other drugs.

A rapid screening system to determine drug affinities for the intestinal dipeptide transporter 2: affinities of ACE inhibitors.

PURPOSE To assess the affinities of a series of ACE inhibitors for the di/tri/oligopeptide transport system (DTS) using a rapid in vitro system. METHODS Monolayers of Caco-2 cells were cultured in plastic wells for 7-9 days and the uptake of Gly-[3H]L-Pro was used as an affinity probe. Gly-[3H]L-Pro (50 nM), together with excess L-Pro (10 mM), to suppress uptake of any [3H]L-Pro produced by degradation of the probe, was incubated with the test compound (usually 1 mM) at pH 6 for 3-mins. The uptake of radiolabel was determined by liquid scintillation counting. RESULTS A 2-dimensional six-domain model of the transporter based on the structure of a phosphinate ACE inhibitor (SQ-29852) was constructed to facilitate interpretation of the competitor affinities. The SQ-29852 molecule was divided into six binding domains (A-F) based on functional groups within these regions and the effects of structural variation in four of these domains (A, C-E) were explored. A series of dipeptide-like compounds varying within specific domains were selected from a large number of commercially available ACE inhibitors and SQ-29852 analogues. Domain A had a preference for an uncharged group, with bulky hydrophobic groups reducing affinity. Domain C exhibited a preference for a positive charge over a neutral function, with the space this functional group occupies contributing to affinity. Domain D favoured lipophilic residues and domain E retained activity when the carboxylic acid was esterified. CONCLUSION The test system is able to reveal structure-activity relationships of peptidomimetic agents and may well serve as a design tool to optimise affinity for the DTS.

Physical Stability and Recrystallization Kinetics of Amorphous Ibipinabant Drug Product by Fourier Transform Raman Spectroscopy

The solid-state physical stability and recrystallization kinetics during storage stability are described for an amorphous solid dispersed drug substance, ibipinabant, at a low concentration (1.0%, w/w) in a solid oral dosage form (tablet). The recrystallization behavior of the amorphous ibipinabant-polyvinylpyrrolidone solid dispersion in the tablet product was characterized by Fourier transform (FT) Raman spectroscopy. A partial least-square analysis used for multivariate calibration based on Raman spectra was developed and validated to detect less than 5% (w/w) of the crystalline form (equivalent to less than 0.05% of the total mass of the tablet). The method provided reliable and highly accurate predictive crystallinity assessments after exposure to a variety of stability storage conditions. It was determined that exposure to moisture had a significant impact on the crystallinity of amorphous ibipinabant. The information provided by the method has potential utility for predictive physical stability assessments. Dissolution testing demonstrated that the predicted crystallinity had a direct correlation with this physical property of the drug product. Recrystallization kinetics was measured using FT Raman spectroscopy for the solid dispersion from the tablet product stored at controlled temperature and relative humidity. The measurements were evaluated by application of the Johnson-Mehl-Avrami (JMA) kinetic model to determine recrystallization rate constants and Avrami exponent (n = 2). The analysis showed that the JMA equation could describe the process very well, and indicated that the recrystallization kinetics observed was a two-step process with an induction period (nucleation) followed by rod-like crystal growth.

Impact of the counterion on the solubility and physicochemical properties of salts of carboxylic acid drugs

Aim: Salt formation is a widely used approach to improve the physicochemical and solid state properties of an active pharmaceutical ingredient. In order to better understand the relationships between the active drug, the selected counterion and the resultant salt form, crystalline salts were formed using four different carboxylic acid drugs and a closely related series of amine counterions. Thirty-six related crystalline salts were prepared, characterized and the relationship between solubility and dissolution behaviour and other properties of the salt and the counterion studied. Methods: Salts of four model acid drugs, gemfibrozil, flurbiprofen, ibuprofen and etodolac were prepared using the counterions butylamine, hexylamine, octylamine, benzylamine, cyclohexylamine, tert-butylamine, 2-amino-2-methylpropan-1-ol, 2-amino-2-methylpropan-1,3-diol andtris(hydroxymethyl)aminomethane. Salt formation was confirmed, the salts were characterized and their corresponding solubilities determined and rationalized with respect to the counterions’ properties. Results and conclusion: The properties of the salt highly dependent on the nature of the counterion and, although there is considerable variation, some general conclusion can be drawn. For the alkyl amines series, increasing chain length leads to a reduction in solubility across all the acidic drugs studied and a reduction in melting point, thus contradicting simplistic relationships between solubility and melting point. Small, compact counterions consistently produce crystalline salts with high melting point accompanied with a modest improvement in solubility and the nature of hydrogen bonding between the ions has a major impact on the solubility.

Factors affecting captopril stability in aqueous solution

Abstract The oxidative and hydrolytic degradation products of captopril, formed under acid and mildly alkaline conditions, have been isolated and identified. The oxidative rate of degradation of captopril over the pH range 2–5.6, in McIlvaines citrate-phosphate buffer, has been studied at 50°C, the rate showing pH-dependency with maximum stability below pH 4.0. The effect of additives such as anti-oxidants, chelating agents and trace metal contaminants, is also presented. The rate of hydrolysis was studied in hydrochloric acid solutions and extrapolated to the pH range of 2–4. The oxidation reaction was the predominant route of degradation over this pH range.

Solid state examination of a gliclazide:beta-cyclodextrin complex

Abstract A solid state complex was formed between the oral sulfonylurea drug gliclazide and beta-cyclodextrin by co-precipitation from a methanol:water co-solvent system. The molar stoichiometry of the complex was determined as 1:2, gliclazide to beta-cyclodextrin, by high-performance liquid chromatography. The complex was examined using differential scanning calorimetry, thermogravimetric analysis, diffuse reflectance infra-red Fourier transform spectroscopy and X-ray diffraction. The results indicate that the molecular structure of the complex involves one molecule of beta-cyclodextrin associating with the benzene ring of the gliclazide molecule and a second beta-cyclodextrin associating with the azabicyclooctyl ring of the gliclazide.