Behavior of decomposition of rifampicin in the presence of isoniazid in the pH range 1-3

Mucoadhesive In Situ Rectal Gel Loaded with Rifampicin: Strategy to Improve Bioavailability and Alleviate Liver Toxicity

Although it is a front-line in tuberculosis treatment, rifampicin (RF) exhibits poor oral bioavailability and hepatotoxicity. Rectal mucoadhesive and in situ rectal gels were developed to overcome drug drawbacks. A RF/polyethylene glycol 6000 co-precipitate was first prepared in different ratios. Based on the drug solubility, the selected ratio was investigated for drug/polymer interaction and then incorporated into in situ rectal gels using Pluronic F127 (15%) and Pluronic F68 (10%) as a gel base and mucoadhesive polymers (HPMC, sodium alginate and chitosan). The formulations were assessed for gelation temperature and gel strength. The selected formulation was investigated for in vivo assessments. The results showed that a 1:1 drug/polymer ratio exhibited satisfying solubility with the recorded drug/polymer interaction. Depending on their concentrations, adding mucoadhesive polymers shifted the gelation temperature to lower temperatures and improved the gel strength. The selected formulation (F4) did not exhibit any anal leakage or marked rectal irritation. Using a validated chromatographic analytical method, F4 exhibited higher drug absorption with a 3.38-fold and 1.74-fold higher bioavailability when compared to oral drug suspension and solid suppositories, respectively. Toxicity studies showed unnoticeable hepatic injury in terms of biochemical, histopathological and immunohistochemical examinations. Together, F4 showed a potential of enhanced performance and also offered lower hepatic toxicity, thus offering an encouraging therapeutic alternative.

Multicomponent crystals of anti-tuberculosis drugs: a mini-review

Tuberculosis (TB) is the leading cause of death from a single infectious agent globally. Some of the early research on TB treatment indicated drug resistance as one of the key challenges in fighting this disease. The discovery that administering two or more drugs simultaneously could lead to much more effective treatment, with reduced drug resistance and shorter periods of chemotherapy, was, therefore, a very significant breakthrough in TB drug research. Pursuant to this discovery, the World Health Organisation (WHO) recommended TB treatment employing fixed-dose combinations (FDCs) containing first line anti-TB drugs; rifampicin, isoniazid, pyrazinamide, streptomycin and ethambutol. Regardless, certain challenges associated with FDCs remain and these include chemical instability and reduced bioavailability of rifampicin. Therefore, some research effort has been directed towards finding ways to deal with these challenges. One such effort involves the use of pharmaceutical co-crystals of the active pharmaceutical ingredients. Consequently, several pharmaceutical co-crystals of isoniazid and pyrazinamide have been reported. This paper aims at reviewing the multicomponent crystal structures of two first-line anti-TB drugs; isoniazid and pyrazinamide. The review will first set out a brief history of the disease, milestones in TB chemotherapy and the challenges associated with current treatment regimens. This will then be followed by a brief introduction to pharmaceutical co-crystals and how they can improve the physical and chemical properties of the active pharmaceutical ingredients. Secondly, multicomponent crystals of the two active pharmaceutical ingredients will be analysed by manual inspection for common supramolecular synthons between the drug molecules as well as between drug molecules and co-formers. Lastly; stability, solubility and dissolution experiments carried out on the pharmaceutical co-crystals of pyrazinamide and isoniazid will be analysed to gain insights into progress made with regards to improving stability and solubility of the active pharmaceutical ingredients.

Anti-tuberculosis site-specific oral delivery system that enhances rifampicin bioavailability in a fixed-dose combination with isoniazid

The in vivo release segregation of rifampicin (RIF) and isoniazid (INH) has been proposed as a strategy to avoid RIF acid degradation, which is known as one of the main factors for reduced RIF bioavailability and can result in drug-resistant tuberculosis. So far, this strategy has been scarcely explored. The aims of this study were to investigate the stability and bioavailability of RIF after combination of a very fast release matrix of RIF with a sustained delivery system of INH. A series of INH-alginic acid complexes (AA-INH) was obtained and characterized. Independent and sequential release profile of AA-INH at biorrelevant media of pH 1.20 and 6.80 was explored. In addition, AA-INH was combined with a RIF-carboxymethylcellulose very fast release complex (CMC-RIF) obtained previously and subjected to acid dissolution assays to evaluate RIF acid stability and determine RIF and INH dissolution efficiencies. Finally, a pharmacokinetic study in dogs was carried out. The AA-INH was easily obtained in solid-state. Their characterization revealed its ionic nature, with a loading capacity of around 30%. The dissolution efficiencies (15 min) confirmed release segregation in acid media with 7.8 and 65.6% for AA-INH and CMC-RIF, respectively. INH release rate from the AA-INH system was slow in acid media and increased in simulated intestinal media. The complete release of INH was achieved after 2 h in simulated intestinal media in the sequential release experiments. The acid degradation of RIF was significantly reduced (36.7%) when both systems were combined and oral administration to dogs revealed a 42% increase in RIF bioavailability. In conclusion, CMC-RIF and AA-INH may be useful for the formulation of a site-specific solid dosage form to overcome some of the main obstacles in tuberculosis treatment. Graphical abstract.

Improved Stability of Rifampicin in the Presence of Gastric-Resistant Isoniazid Microspheres in Acidic Media

The degradation of rifampicin (RIF) in an acidic medium to form 3-formyl rifamycin SV, a poorly absorbed compound, is accelerated in the presence of isoniazid, contributing to the poor bioavailability of rifampicin. This manuscript presents a novel approach in which isoniazid is formulated into gastric-resistant sustained-release microspheres and RIF into microporous floating sustained-release microspheres to reduce the potential for interaction between RIF and isoniazid (INH) in an acidic environment. Hydroxypropyl methylcellulose acetate succinate and Eudragit^® L100 polymers were used for the manufacture of isoniazid-loaded gastric-resistant sustained-release microspheres using an o/o solvent emulsification evaporation approach. Microporous floating sustained-release microspheres for the delivery of rifampicin in the stomach were manufactured using emulsification and a diffusion/evaporation process. The design of experiments was used to evaluate the impact of input variables on predefined responses or quality attributes of the microspheres. The percent degradation of rifampicin following 12 h dissolution testing in 0.1 M HCl pH 1.2 in the presence of isoniazid gastric-resistant sustained-release microspheres was only 4.44%. These results indicate that the degradation of rifampicin in the presence of isoniazid in acidic media can be reduced by encapsulation of both active pharmaceutical ingredients to ensure release in different segments of the gastrointestinal tract, potentially improving the bioavailability of rifampicin.

Encapsulating Rifampicin into SLNs: A Viable Option for Managing its Bioavailability Issues Upon Co-Delivery with Isoniazid

BACKGROUND

Rifampicin is known to degrade at the acidic pH of the stomach, especially in the presence of isoniazid. Although isoniazid also degrades partially, its degradation is reversible.

OBJECTIVE

Presently, we provide a proof of the fact that the simultaneous oral administration of rifampicin (RIF), upon incorporation into solid lipid nanoparticles (RIF-SLNs), with isoniazid (INH) overcomes its INH-induced degradation and improves its oral bioavailability in rats.

METHODS

Solid lipid nanoparticles of RIF (RIF-SLNs) were prepared using a novel and patented method. The effect of INH was investigated on in vivo bioavailability of RIF both in its free and encapsulated (RIF-SLNs) form, after oral administration to rats.

RESULTS

C_max and AUC_0-∞ of RIF increased 158 % and 125 %, respectively, upon incorporation into SLNs versus free RIF when combined with INH. The T_max decreased from 5.67 h to 3.3 h, and the plasma concentration of RIF remained above its MIC (8 μg/ml) at all the tested time points starting with 15 min, when administered as RIF-SLNs in combination with INH.

CONCLUSION

The results confirm the scope of combining RIF-SLNs with INH to overcome the bioavailability of free RIF when combined with INH, especially in fixed dose combinations.

Chitosan-g-oligo(epsilon-caprolactone) polymeric micelles: microwave-assisted synthesis and physicochemical and cytocompatibility characterization

Mucoadhesive chitosan-g-oligo(epsilon-caprolactone) polymeric micelles were synthesized by a microwave-assisted technique and fully characterized in vitro.

Rifampicin-loaded ‘flower-like’ polymeric micelles for enhanced oral bioavailability in an extemporaneous liquid fixed-dose combination with isoniazid

Background: Coadministration of rifampicin (RIF)/isoniazid (INH) is clinically recommended to improve the treatment of tuberculosis. Under gastric conditions, RIF undergoes fast hydrolysis (a pathway hastened by INH) and oral bioavailability loss. Aim: We aimed to assess the chemical stabilization and the oral pharmacokinetics of RIF nanoencapsulated within poly(ε-caprolactone)-b-PEG-b-poly(ε-caprolactone) ‘flower-like’ polymeric micelles. Materials & methods: The chemical stability of RIF was evaluated in vitro under acid conditions with and without INH, and the oral pharmacokinetics of RIF-loaded micelles in rats was compared with those of a suspension coded by the US Pharmacopeia. Results: Nanoencapsulation decreased the degradation rate of RIF with respect to the free drug. Moreover, in vivo data showed a statistically significant increase of RIF oral bioavailability (up to 3.3-times) with respect to the free drug in the presence of INH. Conclusion: Overall results highlight the potential of this nanotechnology platform to develop an extemporaneous liquid RIF/INH fixed-dose combination suitable for pediatric administration.

Original submitted 6 April 2013; Revised submitted 7 August 2013

Nanomedicines in the future of pediatric therapy

Nanotechnology has become a key tool to overcome the main (bio)pharmaceutical drawbacks of drugs and to enable their passive or active targeting to specific cells and tissues. Pediatric therapies usually rely on the previous clinical experience in adults. However, there exists scientific evidence that drug pharmacokinetics and pharmacodynamics in children differ from those in adults. For example, the interaction of specific drugs with their target receptors undergoes changes over the maturation of the different organs and systems. A similar phenomenon is observed for toxicity and adverse effects. Thus, it is clear that the treatment of disease in children cannot be simplified to the direct adjustment of the dose to the body weight/surface. In this context, the implementation of innovative technologies (e.g., nanotechnology) in the pediatric population becomes extremely challenging. The present article overviews the different attempts to use nanotechnology to treat diseases in the pediatric population. Due to the relevance, though limited available literature on the matter, we initially describe from preliminary in vitro studies to preclinical and clinical trials aiming to treat pediatric infectious diseases and pediatric solid tumors by means of nanotechnology. Then, the perspectives of pediatric nanomedicine are discussed.

Novel rifampicin-phospholipid complex for tubercular therapy: synthesis, physicochemical characterization and in-vivo evaluation

To enhance the oral bioavailability of rifampicin (RMP), the newly emerging phospholipid complexation technique was employed. Rifampicin-phospholipid complex (RMP-PC) was prepared by solvent-evaporation method. Infrared spectroscopy (IR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD) and hot stage microscopy (HSM) analysis were employed to confirm the formation of phospholipid complex. The results reveal hydrogen bond formation and electrostatic interaction between RMP and phospholipid molecule play an important role in the formation of RMP-PC without the formation of a new compound. In comparison with the physical mixture and RMP, solubility studies indicated an enhancement in the aqueous solubility of RMP-PC. Stability studies of RMP-PC in presence of isoniazid showed a remarkable improvement of the stability of the phospholipid complex in comparison to free RMP. Oral bioavailability of RMP-PC was evaluated in Sprague-Dawley (SD) rats and plasma rifampicin estimated by LCMS. RMP-PC exhibited higher peak plasma concentration (54.3 vs. 48.5 μg/mL), increased AUC0-∞ (472.4 vs. 147.71 5.812 ± 0.49 μg h/mL), increased T1/2 (8.3 vs. 1.5h) when compared to free RMP implying improved bioavailability of the drug. This enhancement can be attributed to the improvement of the aqueous solubility of rifampicin-phospholipid complex. Hence, phospholipid complexation holds a promising potential for increasing oral bioavailability of poorly water soluble drugs.

Encapsulation of Rifampicin in a solid lipid nanoparticulate system to limit its degradation and interaction with Isoniazid at acidic pH

Rifampicin (RIF), a vital constituent of antitubercular therapy, hydrolyzes at the acidic pH of the stomach. The degradation is further enhanced by its interaction with Isoniazid (INH). Extent of RIF decomposition, in the presence and absence of INH, was determined at pH 1.2 (pH of empty stomach) at 37°C for 4 h (maximum stomach residence time). Both the drugs decomposed at gastric pH (26.5% and 1.43% for RIF and INH respectively). Considering that solid lipid nanoparticles (SLNs) avert drug-drug interaction and also drug degradation, we incorporated RIF into SLNs. Latter reduced its degradation to ~9% (from 26.50% when present alone) and to ~20% (from 48.81% when INH was also present). Subsequent to this, we also incorporated INH into SLNs and the percent degradation of RIF in this combination (RIF SLNs+INH SLNs) further reduced to 12.35%. Furthermore, the degradation of INH in combination with RIF also reduced significantly from 13.2% to 2.7% when both the drugs were encapsulated individually within SLNs. Study therefore highlights the need to develop combinations of antitubercular drugs (ATDs) with caution and also establishes the usefulness of nanoparticulate technology to avoid drug-drug interaction.

Pharmacokinetics, tissue distribution and relative bioavailability of isoniazid-solid lipid nanoparticles

Low levels of isoniazid gain access into plasma following oral administration due to its high aqueous solubility, poor permeability and rapid and extensive hepatic metabolism. Further, a small t(1/2) of 1-4 h indicates its short stay in plasma and the need for repetitive or high doses which may subsequently result in hepatotoxicity and neurotoxicity associated with its use. Isoniazid-solid lipid nanoparticles (SLNs) were prepared to achieve improved bioavailability and prolonged effect, thus minimizing pulsatile plasma concentrations (and associated side effects at peak plasma concentrations). Developed SLNs showed high entrapment efficiency (69%) and small size (d(90) 48.4 nm) such that they are expected to bypass reticulo-endothelial system (RES) pickup resulting in prolonged circulation times and since liver is the major site of metabolism of isoniazid, RES avoidance will reduce its elimination from the body. Single dose (25 mg/kg BW) oral pharmacokinetic studies were performed in plasma and various tissues of rats. A significant improvement (p<0.001) in relative bioavailability in plasma (6 times) and brain (4 times) was observed after administration of isoniazid-SLNs with respect to the free drug solution at the same dose. Insignificant changes in liver concentration coupled with bypass of first pass metabolism and slow release of isoniazid (60%, in 24 h) indicate low incidence of hepatotoxicity. Isoniazid-SLNs showed a 3 times higher LD50.

Preparation of sustained release rifampicin microparticles for inhalation

Objectives

The aim of this research was to develop a novel carrier-free dry powder formulation of rifampicin for inhalation with controlled-release properties.

Methods

Rifampicin dihydrate (RFDH) microcrystals were prepared by a polymorphic transformation of rifampicin. The prepared RFDH microcrystals were coated with poly (dl-lactide-co-glycolide) or poly (dl-lactide), using a spray-dryer equipped with two different types of three-fluid (3F) spray nozzles. The physicochemical and aerodynamic properties of the coated RFDH microcrystals were compared with those of conventional matrix microparticles.

Key findings

The coated RFDH powder, encapsulating 50% of rifampicin, was successfully prepared by simple in-situ coating methods using two different types of 3F nozzles and had mass median aerodynamic diameter values of 3.5–4.5 µm. The thin flaky morphology of RFDH powders, providing good aerosolization properties, was maintained after coating. The coated RFDH formulations showed relatively low initial rifampicin release, compared with the uncoated RFDH crystals, followed by slow rifampicin release (about 70%) over 8 h in phosphate-buffered saline media (pH 7.4). Significant chemical degradations were not observed from the crystalline-structured RFDH formulations, while the amorphous-structured matrix formulations showed chemical degradation in six months.

Conclusions

These polymer coated RFDH formulations may be a valuable alternative in the treatment of tuberculosis since the carrier-free formulation offers the benefit of delivering a maximum-potency formulation of the antibiotic directly to the site of infection, and long drug residence times may be achieved by the controlled release of the drug.

Predicting the oral absorption of a poorly soluble, poorly permeable weak base using biorelevant dissolution and transfer model tests coupled with a physiologically based pharmacokinetic model

For predicting food effects and simulating plasma profiles of poorly soluble drugs, physiologically based pharmacokinetic models have become a widely accepted tool in academia and the pharmaceutical industry. Up till now, however, simulations appearing in the open literature have mainly focused on BCS class II compounds, and many of these simulations tend to have more of a "retrospective" than a prognostic, predictive character. In this work, investigations on the absorption of a weakly basic BCS class IV drug, "Compound A", were performed. The objective was to predict the plasma profiles of an immediate release (IR) formulation of Compound A in the fasted and fed state. For this purpose, in vitro biorelevant dissolution tests and transfer model experiments were conducted. Dissolution and precipitation kinetics were then combined with in vivo post-absorptive disposition parameters using STELLA® software. As Compound A not only exhibits poor solubility but also poor permeability, a previously developed STELLA® model was revised to accommodate the less than optimal permeability characteristics as well as precipitation of the drug in the fasted state small intestine. Permeability restrictions were introduced into the model using an absorption rate constant calculated from the Caco-2 permeability value of Compound A, the effective intestinal surface area and appropriate intestinal fluid volumes. The results show that biorelevant dissolution tests are a helpful tool to predict food effects of Compound A qualitatively. However, the plasma profiles of Compound A could only be predicted quantitatively when the results of biorelevant dissolution test were coupled with the newly developed PBPK model.

Drug-Drug Interaction Studies on First-Line Anti-tuberculosis Drugs

The purpose of this study was to carry out drug-drug compatibility studies on pure first line anti-tuberculosis drugs, viz., rifampicin (R), isoniazid (H), pyrazinamide (Z), and ethambutol hydrochloride (E). Various possible binary, ternary, and quaternary combinations of the four drugs were subjected to accelerated stability test conditions of 40 degrees C and 75% relative humidity (RH) for 3 months. For comparison, parallel studies were also conducted on single drugs. Changes were looked for in the samples drawn after 15, 30, 60, and 90 days of storage. Analyses for R, H, and Z were carried out using a validated HPLC method. The E was analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS), as it does not absorb in ultraviolet (UV). All single pure drugs were relatively stable and showed only 3%-5% degradation under accelerated conditions for 3 months. However, significant interactions were observed in case of the drug mixtures. In particular, ternary and quaternary drug combinations containing R and H along with Z and/or E were very unstable, showing 90%-95% and 70%-75% loss of R and H, respectively. In all these cases, isonicotinyl hydrazone (HYD) of 3-formylrifamycin and H was found to be the major degradation product. In case of RE and RZE mixtures, where H was absent, 3-formylrifamycin was instead the key degradation product. Another unidentified peak was observed in the mixture containing RZE. Apart from these chemical changes, considerable physical changes were also observed in pure E and the mixtures containing E, viz., RE, ZE, RHE, RZE, and RHZE. In addition, significant physical changes associated with noteworthy loss of H and E were also observed in mixtures containing HE and HZE. The present study thus amply shows that the four primary anti-tuberculosis drugs, when present together, interact with each other in a multiple and complex manner.

Formulation and Statistical Optimization of a Novel Crosslinked Polymeric Anti‐Tuberculosis Drug Delivery System

The crux of this research was the pragmatic investigation into the formulation of a reconstitutable multiparticulate anti-tuberculosis drug delivery system for facilitated administration for the attainment of segregated gastrointestinal (GI) delivery of rifampicin (RIF) and isoniazid (INH) in order to address issues of unacceptable RIF bioavailability on coadministration with INH. Ionotropically crosslinked polymeric enterospheres for delivery of INH to the small intestine were developed via a response surface methodology for the design and optimization of the formulation and processing variables. A 3(4) Box-Behnken statistical design was constructed. The concentration of zinc sulfate salting-out and crosslinking electrolyte, the crosslinking reaction time, the drying temperature (DT), and the concentration of triethyl citrate plasticizer were varied for determination of their effect on the molar amount of zinc (n(Zn)) incorporated in the crosslinked enterosphere, drug entrapment efficiency (DEE), and mean dissolution time (MDT) at t(2h) in acidic media (0.1 M HCl). Complexometric determination of zinc cations (Zn(2+)) revealed that 23.70-287.89 mol of Zn(2+) per mole of polymer were implicated in crosslink formation. DEE of 27.92% to 99.77% were obtained. Drug release at t(2h) ranged from 1.67% to 73.04%. The salting-out and crosslinking agent significantly affected n(Zn) (p = 0.034) and the DEE (p = 0.000), as did the concentration of plasticizer employed (p = 0.000 and 0.002, respectively). High DTs (>42.5 degrees C) also significantly improved DEE (p = 0.029). ZnSO(4) had a significant effect on the MDT (p = 0.000). A dry dispersible multiparticulate system incorporating the optimally designed INH-loaded enterospheres and RIF was developed. Bivariate regression analysis of UV spectrophotometric absorbance data allowed in vitro resolution of RIF and INH release at simulated gastric pH.

Tuberculosis chemotherapy: current drug delivery approaches

Tuberculosis is a leading killer of young adults worldwide and the global scourge of multi-drug resistant tuberculosis is reaching epidemic proportions. It is endemic in most developing countries and resurgent in developed and developing countries with high rates of human immunodeficiency virus infection. This article reviews the current situation in terms of drug delivery approaches for tuberculosis chemotherapy. A number of novel implant-, microparticulate-, and various other carrier-based drug delivery systems incorporating the principal anti-tuberculosis agents have been fabricated that either target the site of tuberculosis infection or reduce the dosing frequency with the aim of improving patient outcomes. These developments in drug delivery represent attractive options with significant merit, however, there is a requisite to manufacture an oral system, which directly addresses issues of unacceptable rifampicin bioavailability in fixed-dose combinations. This is fostered by the need to deliver medications to patients more efficiently and with fewer side effects, especially in developing countries. The fabrication of a polymeric once-daily oral multiparticulate fixed-dose combination of the principal anti-tuberculosis drugs, which attains segregated delivery of rifampicin and isoniazid for improved rifampicin bioavailability, could be a step in the right direction in addressing issues of treatment failure due to patient non-compliance.

Study of the interaction between rifapentine and isoniazid under acid conditions

A well-known problem of anti-tuberculosis fixed-dose combination (FDC) products containing rifampicin (R) and isoniazid (H) is the fall in bioavailability, in particular of R, when two or more drugs are present together. The same has been ascribed to hydrolysis of R to 3-formylrifamycin (3-RIF) under stomach acid conditions and reaction of the latter with H to form isonicotinyl hydrazone (HYD). The objective of present study was to explore whether the same reaction occurred when H was present along with rifapentine (Rp), a newer long acting rifamycin, which is structurally similar to R. Clinical trials are currently undergoing for co-administration of Rp with H in patients who had completed 2 months of standard chemotherapy. For the purpose, first a validated HPLC method was developed for the separation of Rp and H, and the same was used for the study of interaction between the two drugs. Like R, Rp was also found to convert to 3-RIF in acid conditions, which reacted further with H to form HYD. The pH-rate profile was also similar in shape to that established with the combination of R and H; maximum decomposition occurred at pH 2, where Rp loss was to an extent of approximately 30%, while corresponding decomposition of H was approximately 9%. These values were similar to those reported for the combination of R (approximately 33%) and H (approximately 10%). Hence, the study suggests that co-administration of Rp and H should be avoided, like in case of R and H, and the two drugs should not be formulated directly into a single dosage form.

Mechanistic explanation to the catalysis by pyrazinamide and ethambutol of reaction between rifampicin and isoniazid in anti-TB FDCs

Rifampicin and isoniazid are known to interact with each other in solid formulation environment to yield isonicotinyl hydrazone (HYD). In earlier studies, this reaction was indicated to be catalyzed by pyrazinamide and ethambutol hydrochloride, the two other co-drugs present in oral anti-tuberculosis fixed-dose combination (FDC) formulations. Accordingly, the present study was carried out to understand the catalytic role of pyrazinamide and ethambutol hydrochloride on the reaction between rifampicin and isoniazid. For the purpose, organic bases and amides similar in structure to pyrazinamide and ethambutol hydrochloride were combined individually with rifampicin and isoniazid. The compounds employed were pyrazine, piperdine, pyrollidine, pyridine, triethylamine, diisopropylethylamine, picolinamide, benzamide, ethylenediamine, ethanolamine, diethanolamine, and triethanolamine. An additional study was also carried out in the presence of free base of ethambutol. The mixtures were exposed to accelerated stability test condition of 40 degrees C/75% RH for 15 d. The nature of the products formed and the changes in relative concentrations of the drugs and products were followed by HPLC. The drugs showed different extent of degradation, yielding HYD, and in some cases degradation products of rifampicin. The results confirmed the catalytic role of pyrazinamide and ethambutol hydrochloride. The catalysis is postulated to involve intra-molecular proton transfer during transhydrazone formation process, entailing a tetrahedral mechanism.

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.

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.