Behavior of moisture gain and equilibrium moisture contents (EMC) of various drug substances and correlation with compendial information on hygroscopicity and loss on drying.

The behavior of moisture gain and equilibrium moisture content (EMC) was determined for 30 drug substances at relative humidities (RH) ranging from 11% to 93%. Based on the results, the drugs were categorized into different classes, following the classification system proposed by Callahan et al. About 23 fell under Class-I (nonhygroscopic), four under Class-II (slightly hygroscopic), and three under Class-III (moderately hygroscopic). Most of the Class-III drugs converted from solid to liquid state at > 75% RH, and the moisture increase was more than 40% above 90% RH. However, the moisture increase was comparatively much smaller at humidities < 40%-50%. Hence, no drug could be categorized in Class-IV (very hygroscopic), where gain of moisture is generally higher even at lower humidities. The results were correlated to the statements given in the compendia on hygroscopicity, and also the values of loss on drying (LOD). The study suggests that there is a need to rationalize the pharmacopoeial information.

High-performance liquid chromatographic method for the simultaneous estimation of the key intermediates of duloxetine.

A simple reversed-phase high-performance liquid chromatographic method employing C-18 column has been developed for simultaneous analysis of three intermediates in the synthesis of S-duloxetine, the antidepressant drug, viz., 2-acetyl thiophene (AT), N,N-dimethyl-3-keto-(2-thienyl)-propanamine (DKTP) and (S)-N,N-dimethyl-3-hydroxy-(2-thienyl)-propanamine (DHTP). Good separations were achieved by employing an isocratic system using acetonitrile and 0.05M phosphate buffer (pH 7.0) containing 0.02% diethylamine. The detection was carried out at 241nm. The method was validated for linearity, range, accuracy and precision. The developed method was applied for monitoring the progress of chemical synthesis of DKTP from AT followed by the biocatalytic reduction of DKTP to DHTP as the disappearance of the substrate and formation of the product can be monitored simultaneously by the present method.

Overestimation of rifampicin during colorimetric analysis of anti-tuberculosis products containing isoniazid due to formation of isonicotinyl hydrazone.

When present together in fixed-dose combinations (FDC) of anti-tuberculosis drugs, rifampicin (R) and isoniazid (H) interact with each other to form isonicotinyl hydrazone (HYD). In a preliminary study, this product was found to possess similar colorimetric spectrum to that of rifampicin. Therefore, an investigation was undertaken to determine interference of HYD during analysis of rifampicin in FDC products by colorimetry. For the purpose, standard plots were constructed for rifampicin and HYD at 475 nm, the wavelength maximum for both the compounds. The plots were linear in the range of 10-100 microg/ml. Molar absorptivity values for rifampicin and HYD were 15279 and 5034, respectively. It indicated that HYD possessed one-third absorptivity to that of rifampicin. The analysis of combinations of rifampicin and HYD revealed that rifampicin could be overestimated to a maximum extent of 33%, while interference varied at other relative ratios of the two compounds. This was also confirmed by colorimetric and HPLC analysis of a degraded marketed product and samples from a dissolution study. Thus this investigation suggests that any method devoid of interference of HYD should be preferred for analysis of rifampicin, whenever it is present along with isoniazid.

Interference of isonicotinyl hydrazone in the microbiological analysis of rifampicin from anti-tuberculosis FDC products containing isoniazid.

Microbiological assay is a sensitive method for the estimation of rifampicin (R). In the present study, interference due to isonicotinyl hydrazone (HYD), an interaction product of R and isoniazid (H), was checked during microbiological analysis of R, employing Bacillus subtilis and Sarcina lutea. The assays were done by disc diffusion method. Both R and HYD showed linear log response curves in the range of 0.01-10microg. In the presence of HYD, R was overestimated when tested against S. lutea and underestimated in case of B. subtilis. The same extent and type of interference was observed on assay of a marketed anti-tuberculosis fixed-dose combination product, subjected to accelerated stability testing (40 degrees C/75% RH) for 1 month. This means that response of organisms used in microbiological assay of R might vary in the presence of HYD, with possibility of incorrect conclusions. Therefore, the study suggests that before a microbiological method involving a particular organism is extended to the determination of R in FDC formulations containing H, it should be tested for the influence of HYD and used only if non-interfering.

The physical and chemical stability of anti-tuberculosis fixed-dose combination products under accelerated climatic conditions.

OBJECTIVE: To determine the physical and chemical stability of anti-tuberculosis fixed-dose combinations (FDC) of rifampicin (RMP), isoniazid (INH), pyrazinamide (PZA) and ethambutol (EMB) sold on the Indian market. METHODS: The products were stored for 3 months under ICH/WHO accelerated conditions (40 degrees C / 75% RH), with and without the original packaging in the presence and absence of light. RESULTS: The initial RMP, INH and PZA content was found to be within the range of 90-110% of the label claim. However, the products were found to have some chemical instability even initially; one of the tablets also showed physical instability. Under accelerated conditions, the unpackaged products underwent severe changes, whereas both physical and chemical changes were also observed in the packaged formulations. The physical changes were stronger under lighted conditions. A significant finding is that PZA and perhaps EMB may play a catalytic role in the interaction between INH and RMP. CONCLUSION: This study suggests that, unless they are packed in barrier packaging, anti-tuberculosis FDC formulations should be considered unstable, and due consideration should be given to their development pharmaceutics, packaging and stability testing.

An explanation for the physical instability of a marketed fixed dose combination (FDC) formulation containing isoniazid and ethambutol and proposed solutions.

An investigation was carried out to explore the possible reason for the physical instability of a marketed strip packaged anti-TB fixed dose combination (FDC) tablet containing 300 mg of isoniazid (H) and 800 mg of ethambutol hydrochloride (E). The instability was in the form of distribution of white powder inside the strip pockets. High-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS-MS) studies confirmed that both H and E were present in the powder. The same was also confirmed through Fourier-transform infrared (FTIR) spectroscopy, which also indicated absence of interaction between the two drugs. No sublimation of the drugs was observed up to 110 degrees C, indicating that the observed instability was not due to this reason. Subsequently, attention was paid to the possibility of moisture gain by the tablets through defective packaging (which was established) due to hygroscopicity of E. To understand the phenomenon further, pure drugs and their mixtures were stored under accelerated conditions of temperature and humidity [40 degrees C/75% relative humidity (RH)] and both increase in weight and physical changes were recorded periodically. The mixtures gained moisture at a higher rate than pure E and those with higher content of E became liquid, which on withdrawal from the chambers, became crystallized. The drug mixture containing H:E at a ratio of 30:70 w/w, which was similar to the ratio of the drugs in the tablets (27:73 w/w), crystallized fastest, indicating formation of a rapid crystallizing saturated system at this ratio of the drugs. It is postulated that the problem of instability arises because of the formation of a saturated layer of drugs upon moisture gain through the defective packaging material and drying of this layer with time. The study suggests that barrier packaging free from defects and alternatively (or in combination) film coating of the tablets with water-resistant polymers are essential for this formulation.

Evidence of efflux-mediated and saturable absorption of rifampicin in rat intestine using the ligated loop and everted gut sac techniques.

This study was carried out to explore whether efflux-mediated and saturable mechanisms play any role toward poor and variable intestinal absorption of rifampicin. In situ segmental permeability of rifampicin at various residence times was determined in rat gastrointestinal tract using the ligated loop technique. The involvement of efflux-mediated and saturable absorption of rifampicin was studied in rat intestine using the everted sac method. The samples were analyzed by a validated HPLC method. Rifampicin showed decreased permeability in jejunum and ileum with an increase in residence time. The permeation of rifampicin from the serosal to the mucosal side (secretion) was significantly higher than permeation from the mucosal to the serosal side (absorption) of jejunum and ileum. This indicated the involvement of efflux-mediated transport. Addition of verapamil, an inhibitor for P-glycoprotein (Pgp), multidrug resistance associated protein-2 (MRP-2), and other related transporters, increased absorption of rifampicin in jejunum and ileum by 2-3-fold and decreased secretion by almost 4-fold. The permeation rate (flux) of rifampicin through duodenum increased with concentration up to 300 microg/mL, becoming constant thereafter, indicating the existence of saturable absorption. There was no saturable permeation in jejunum and ileum. Thus the present study indicates the involvement of efflux-mediated and saturable absorption mechanisms of rifampicin in rat intestine, which act as barriers to the absorption of the drug. This explains the drug's poor absolute bioavailability. As Pgp varies from person to person to an extent of 2-8-fold, it can be one direct reason for the interindividual variable bioavailability shown by rifampicin.

Regional gastrointestinal permeability of rifampicin and isoniazid (alone and their combination) in the rat.

OBJECTIVE: To determine if rifampicin (RMP) and isoniazid (INH) are absorbed from different gastrointestinal tract (GIT) sites, and to ascertain the feasibility of producing new fixed-dose combination (FDC) products containing RMP and INH by segregating drug delivery at different sites along the GIT. DESIGN: Permeability of RMP and INH (alone and in combination) was determined in various segments of rat GIT (stomach, duodenum, jejunum and ileum) at concentrations of respectively 2.4 and 1.2 mg/ml, using a ligated loop technique. Drug analysis was performed by HPLC. Extent of absorption was considered as the total drug disappearing from the loop. Permeability was correlated with solubility and decomposition data at pH corresponding to different GIT sites. RESULTS: RMP was well absorbed from the stomach due to its solubility, which was maximum between pH 1-2. INH was poorly absorbed from the stomach, but was well absorbed from all three segments of the intestine. In combination, RMP disappearance was enhanced in the presence of INH in the stomach and jejunum, but INH disappearance was not influenced by RMP. CONCLUSION: The study shows higher in situ RMP disappearance in the presence of INH, attributable to drug degradation due to catalysis by INH. As the two drugs show regional specific permeability, FDCs without reduced RMP bioavailability resulting from its decomposition in the presence of INH can be designed by segregating delivery of the two drugs by around 3-4 h. RMP should be released in the stomach and INH in the intestine.

Behavior of uptake of moisture by drugs and excipients under accelerated conditions of temperature and humidity in the absence and the presence of light. 1. Pure anti-tuberculosis drugs and their combinations.

An investigation was carried out to determine the behavior of moisture gain by four anti-tuberculosis drugs, viz. rifampicin, isoniazid, pyrazinamide and ethambutol, when exposed in pure form and in combinations to accelerated conditions of 40 degrees C and 75% RH, in the absence and the presence of light. Weight gain was seen only in those samples that contained ethambutol, and this behavior was observed both in dark and lighted chambers. There was a decrease in moisture uptake with an increase in the number of drugs in the mixture. Another observation was a higher weight gain by the mixture of ethambutol and isoniazid in a dark chamber, than either pure ethambutol or drug combinations containing ethambutol. The most interesting finding was an overall acceleration of weight gain in the presence of light as compared with dark conditions, which is a hitherto unknown phenomenon.

A critical review of the probable reasons for the poor variable bioavailability of rifampicin from anti-tubercular fixed-dose combination (FDC) products, and the likely solutions to the problem.

The problem of poor/variable bioavailability of rifampicin, which is shown in particular when the drugs are present in anti-tubercular fixed-dose combination (FDC) products, is a matter of serious concern. There is a potential of failure of therapy in patients with an active disease. It perhaps also is a contributory factor towards the increasing resistance to anti-tubercular drugs. Unfortunately, the origin and cause of the problem is not clearly understood, though GMP and crystalline changes in the drug are invariably cited as the principal reasons. In this write-up, various probable physical and/or chemical reasons are critically reviewed. The enhanced decomposition of rifampicin in the presence of isoniazid in stomach after ingestion is indicated to be the key factor behind the problem. Some simple solutions offered by the knowledge of the cause are discussed and it is concluded that there is a need to have a multifaceted approach to handle the problem.