pH-dependent dissolution rate of nitrofurantoin from commercial suspensions, tablets, and capsules

An audit of nitrofurantoin use in three Australian hospitals

BACKGROUND

International guidelines have recommended the long-acting formulation of nitrofurantoin as first-line treatment for uncomplicated urinary tract infections (UTIs) since 2010. Australian guidelines have only recently listed nitrofurantoin as a first-line agent, but the long-acting formulation is not available. In the setting of increasing multidrug-resistance, the unavailability of the long-acting formulation of nitrofurantoin in Australia, and anecdotal perception of confusion regarding dosing, we audited nitrofurantoin use.

METHODS

We performed a retrospective audit of nitrofurantoin use at Alfred Health. All patients dispensed nitrofurantoin from January 2016 to June 2018, as identified from pharmacy dispensing records, were eligible. We used a standardised case report form to extract data from medical records, including dosing regimen and indication.

RESULTS

We included 150 patients with 151 nitrofurantoin prescriptions in the analysis, of whom 74% [111/150] were female. Nitrofurantoin was most commonly dispensed for the treatment of UTIs (68% [103/151] versus 32% [48/151] for UTI prophylaxis). For the treatment of uncomplicated UTIs, the most frequently used dose was 100 mg twice daily for five days. In male patients, the 100 mg twice daily for seven days was the most popular regimen. The prophylactic dose of 50 mg once daily was used in women but rarely in men. We did not find evidence of dose adjustment for renal impairment.

CONCLUSION

While treatment duration was consistent with guidelines, the dosage and frequency used was often incorrect for the formulation and was not adjusted for renal function. Nitrofurantoin use is likely to increase, so clarification regarding optimal nitrofurantoin dosing regimens may be appropriate.

Review of the pharmacokinetic properties of nitrofurantoin and nitroxoline

Nitrofurantoin and nitroxoline are oral antibiotics for the treatment or prophylaxis of acute urinary tract infections. New interest in both these drugs is increasing because of the emergence of resistance to other antibiotics, but knowledge of their pharmacokinetics (PK) is lacking since they were developed before the advent of standardized research for drug approval. The aims of this review were to (i) summarize the PK data reported in the literature and (ii) to identify PK knowledge gaps. The current body of PK knowledge of both drugs appears to be poor and mainly based on old studies. Nitrofurantoin PK values were obtained from studies using many variables, e.g. formulations, crystal sizes and analytical methods, resulting in high interindividual variability in PK parameters and no uniform PK profile. Clinical experience and PK data for nitroxoline are even more limited since the drug is registered in only Germany and a few (Eastern European) countries. Clinical studies in relevant patient populations are needed with commercially available nitrofurantoin and nitroxoline formulations at approved dosing regimens to more fully characterize their PK profiles, and to investigate the influence of patient characteristics on these profiles in order to optimize efficacy and avoid toxicity and emergence of resistance. Only with this updated knowledge and efficacy data from well-structured trials can both drugs maintain their antimicrobial activity against uropathogens.

Modelling, solubility and pK(a) of five sparingly soluble drugs

Drug solubility is an important aspect of drug development. The objective of this investigation was to measure solubilities of five drugs (cimetidine, phenylbutazone, fenbufen, nitrofurantoin, triamterene) at constant pH in range of temperature from 270 to 340K in three solvents: water, ethanol and 1-octanol with the dynamic-visual method and the saturation shake-flask method using spectrophotometric analysis. The Barton group contribution method was used for the calculations of molar volumes of solutes. The thermodynamic description of the solubility curves was made using the thermophysical properties obtained with the differential scanning microcalorimetry technique (DSC). The DSC measurements have shown different than existing in the literature enthalpies of melting for phenylbutazone and fenbufen. The experimental solubility data also differ from the literature data, normally measured at one, or two temperatures only. The solubility data have been correlated by means of three commonly known excess Gibbs energy, G(E) equations. The activity coefficients of drugs at saturated solutions were calculated from the experimental data. Reexamination of the pK(a) values using diluted solutions was made with the Bates-Schwarzenbach method for the pK(a) measurements. The association constants and corresponding pK(a) values of drugs were close to the most of the literature data. We hope that our new solubility data, thermophysical data, and pK(a) values will improve all prediction-methods and their precision.

Comparative evaluation of mean particle size of bulk drug powder in pharmaceutical preparations by Fourier-transformed powder diffuse reflectance infrared spectroscopy and dissolution kinetics

This paper reports a quantitative correlation between specific FT-IR absorption peak intensity of phenytoin (PTH) to its particle size in the bulk as well as in powder blends. Absorption peak intensity in FT-IR spectra of phenytoin (PHT) increased with decrease of mean particle size of the powders. The relationships between the FT-IR absorbance at 1724 cm-1 of PHT and the specific surface area (Sw) obtained by the BET N2 gas absorption method or the reciprocal of mean particle size (D50) obtained by particle counting of SEM photomicrographs of ground samples showed straight lines, indicating that the absorbance followed the equation log (lo/l) = K1/D50 = K2Sw, where lo and l are intensities of the incident and transmitted light, respectively, and K1 and K2 are constants. In binary mixture of powders containing 50% PHT and 50% lactose, the relationship between the absorbance at 1724 cm-1 of PHT powder and Sw of the reciprocal of D50 also gave a linear plot. The dissolution behaviors of the binary mixture samples were measured in pH 1.2 buffer containing 0.1% sodium lauryl sulfate at 37 degrees C. The data from initial rates of dissolution of the binary mixture was utilized to calculate the particle size (Dg) using the Hixon-Crowell equation, and these particle size values were consistent with those obtained from Sw data as well as the absorbance data by the IR spectral method.

3(3) factorial design-based optimization of the formulation of nitrofurantoin microcapsules

A microcapsule form of nitrofurantoin was prepared by a simple coacervation method with carboxymethylcellulose and aluminium sulfate. 3(3) factorial design was performed for three independent variables, namely, the particle size of the drug, the size of the microcapsules and the pH of the dissolution medium. The dissolution tests with the formulated microcapsules were carried out according to the United States Pharmacopeia XXII rotating basket method at pH 1.2, 5, and 7.5, which represent the pH of gastrointestinal fluids. Release data were examined kinetically and the ideal kinetic models were estimated and t(63.2) values obtained from RRSBW distribution were used in the factorial design experiment. The influence of the independent variables on the dissolution of nitrofurantoin microcapsules could be expressed as the pH of the dissolution medium > particle size of the microcapsule > particle size of nitrofurantoin. The other aim of this study was to evaluate microcapsule formulation in terms of the United States Pharmacopeia criteria with a minimum of experiments. Our findings suggest that dosage forms which comply with the pharmacopoeia criteria for dissolution can be prepared and selected by factorial design.

In vitro and in vivo evaluation of seven 50 mg and 100 mg nitrofurantoin tablets

Four 50 mg and three 100 mg marketed nitrofurantoin tablets were studied in 14 healthy male subjects. Urine was collected 1, 2, 3, 4, 6, 8, 12, and 23 h after each dose, and nitrofurantoin was assayed by HPLC. The in vitro dissolution of the tablets was determined using USP Apparatus 1 and 2, with 0.1 N hydrochloric acid and pH 7.2 buffer as the dissolution fluids. One of the 50 mg tablets was more rapidly and completely absorbed than the other six products. The incidence of side-effects for this product was as low or lower than the other products. It was determined that the use of the USP Apparatus 1, at 100 rev min-1, with sampling of the pH 7.2 fluid at 30 min, provided for the best overall relationship between the urinary excretion and in vitro dissolution.

Studies on freeze-dried drug-milk formulations. II: Effect of regenerated fluid volume on nitrofurantoin bioavailability

The effect of different milk volumes on the extent and consistency of nitrofurantoin (1-[(5-nitrofurfurylidene)amino]hydantoin) absorption from freeze-dried nitrofurantoin-milk formulations was studied in four male volunteers in three separate crossover designs. Each volunteer received six single-dose treatments (one 100-mg nitrofurantoin capsule with 100, 200, and 400 mL of milk and 100 mg of nitrofurantoin as a freeze-dried nitrofurantoin milk formulation regenerated with 100, 200, and 400 mL of water). Analysis of the urine data revealed superiority of the nitrofurantoin-milk formulations regenerated with 200 and 400 mL of milk over the corresponding capsule formulations in the rates and extents of nitrofurantoin excretion. The binding of nitrofurantoin to casein and bovine serum albumin and its solubility in the presence of the proteins were measured in vitro. The presence of both proteins caused increases in the solubility of nitrofurantoin. Normal protein binding is responsible for the increase of nitrofurantoin solubility in the presence of bovine serum albumin, whereas the increase of nitrofurantoin solubility in the presence of casein is attributed to the formation of aggregates in casein solution at 37 degrees C. The in vivo data were discussed in light of the in vitro data. The freeze-dried nitrofurantoin-milk formulation regenerated with 200 mL of water has a potential for use as a nitrofurantoin delivery system.

Nonlinear assessment of nitrofurantoin bioavailability in rabbits

The influence of route of administration on the absolute bioavailability and GI tract absorption of nitrofurantoin was investigated in rabbits. The disposition of nitrofurantoin was described by a one-compartment model with simultaneous first-order and Michaelis-Menten type elimination kinetics, and bioavailability was estimated by nonlinear assessment. The plasma levels following oral administration were significantly lower than those after intravenous administration, and absolute F values for oral administration were approximately 0.3. However, F values following intraduodenal administration and portal vein infusion were nearly unity, and it was concluded that the reduction of bioavailability following oral administration could not be attributed to metabolism by intestinal microflora or to the hepatic first-pass effect. Thus, reduction of F values following oral administration is probably due to gastric degradation of the drug. The effects of factors influencing bioavailability, such as water volume taken with the drug, change of gastric emptying rate and effect of particle size, were also investigated. Increase of volume of water administered tended to improve the bioavailability, and a particle size dependency was also observed.

Bioavailability and dissolution behavior of trisulfapyrimidine suspensions

The bioavailability of seven commercial trisulfapyrimidine suspensions was studied in 14 adult male volunteers. Fifteen blood samples were collected over a 48-hr period following administration of a 1-g dose of each suspension. Serum was assayed for each component (sulfadiazine, sulfamerazine, and sulfamethazine) by high-pressure liquid chromatography. Analysis of variance indicated several significant differences among the seven commercial preparations with respect to Cmax Tmax, and AUC for sulfadiazine, sulfamerazine, and sulfamethazine, The in vitro behavior of each suspension was then studied by the paddle method of the Food and Drug Administration. A 0.5-ml sample was introduced into 900 ml of hydrochloric acid (2.2 x 10(-4) M) at 37 degree and dissolved using a paddle speed of 25 rpm. Samples withdrawn at 15 and 30 min were analyzed by high-pressure liquid chromatography, and the percent of sulfadiazine, sulfamerazine, and sulfamethazine was calculated. Significant correlation was obtained between an in vivo parameter (Cmax for sulfadiazine) and an in vitro parameter (percent sulfadiazine dissolved in 30 min). Results indicate that this method is suitable for the in vitro screening of trisulapyrimidine suspensions.