This review article recapitulates the analytical methods for the quantitative determinations of diltiazem and three H2 receptor antagonists (cimetidine, ranitidine, and famotidine) by one of the spectroscopic technique (UV spectrophotometery) and separation technique such as high-performance liquid chromatography (HPLC). The clinical and pharmaceutical analysis of these drugs requires effective analytical procedures for quality control, pharmaceutical dosage formulations, and biological fluids. An extensive survey of the literature published in various analytical and pharmaceutical chemistry-related journals has been compiled in its review. A synopsis of reported spectrophotometric and high-performance liquid chromatographic methods for individual drug is integrated. This appraisal illustrates that majority of the HPLC methods reviewed are based on the quantitative analysis of drugs in biological fluids, and they are appropriate for therapeutic drug monitoring purpose.

1. Introduction

1.1. Diltiazem Hydrochloride

Diltiazem hydrochloride (DLZ), d-cis diltiazem, d-cis-3-acetyloxy-5-[2-(dimethylamino) ethyl]-2,3-dihydro-2-(4-methoxyphenyl)-1,5-benzothiazepin-4(5H)one)hydrochloride (Figure 1) are one of the widely used benzothiazepine Ca2+-blocking drugs (calcium channel blockers). Clinically it is used to treat cardiovascular diseases such as angina pectoris, hypertension, and cardiac arrhythmias [1, 2].

1.2. Histamine H2 Receptor Antagonists

Histamine H2 receptor antagonists: cimetidine (CIM), ranitidine (RAN), and famotidine (FAM) classified as class III drugs (high solubility, low permeability) according to the Biopharmaceutics Classification System (BCS) [3, 4] are used in the treatment of gastrooesophageal reflux disease and gastric and duodenal ulceration [5].

1.3. Cimetidine

Cimetidine  is N-Cyano-N-methyl-N-[2-[[(5-methyl-1H-imidazol-4-yl)methyl]thio]ethyl]guanidine, contains imidazole ring (Figure 2), and inhibits hepatic cytochrome P450 (CYP) 1A2, 2C9, 2D6, and 3A4 P450 isoforms [6].

1.4. Ranitidine Hydrochloride

Ranitidine  hydrochloride  is N-{2-[[[5-[(dimethylamino)methyl]-2 furanyl]methyl]thio]ethyl}-N′-methyl-2-nitro-1,1-ethenediamine (Figure 3), a histamine H2 receptor antagonist with a furan ring structure that increases its potency to inhibit gastric acid secretion induced by various stimuli, while lacking the antiandrogenic and hepatic microsomal enzyme-inhibiting effects [7].

1.5. Famotidine

Famotidine, 3-[[[2-[(aminoiminomethyl) amino]-4-thiazolyl]methyl]thio]-N (aminosulfonyl) (Figure 4), is highly selective histamine H2 receptor antagonist, indicated for the treatment of duodenal ulcer, gastric ulcer, gastroesophageal reflux disease, and Zollinger-Ellison syndrome [8], in the treatment of Parkinson’s and inhibits basal and nocturnal gastric secretion as well as secretion stimulated by food and pentagastrin [9].

The present review article deals with the analytical methods that are useful for pure form, pharmaceutical formulations, and biological specimens of diltiazem and H2 receptor antagonists (cimetidine, ranitidine, and famotidine) were focused and summarized.

2. Analytical Methods

Assay method development is very significant for pharmaceutical industries and pathological laboratories, and this is always desirable to select and develop simple, accurate, precise, and economical method for the determination of drugs in pharmaceutical dosage forms and pathological samples. Analytical data are used to screen potential drugs in biological samples, aid in the development of drug syntheses, support formulation studies, monitor the stability of bulk pharmaceuticals and formulated products, and test final products for release [10].

3. Official and Compendial Methods of Analysis

Monographs in the “Analytical Profile of Drug Substances” series were also published for diltiazem [11], cimetidine [12], and ranitidine [13]. Eur Ph 2002 [14] recommends an HPLC assay for diltiazem in its dosage forms. Stability-indicating method for the determination of diltiazem hydrochloride in formulated oral preparations by the HPLC technique has been recommended in USP 29 [15]. High-performance liquid chromatographic methods are prescribed according to B.P 2011 and USP 34 for quantitative analysis of pharmaceuticals containing cimetidine, ranitidine hydrochloride, and famotidine [1620]. Chromatographic conditions of official methods are presented in Table 1. However, British Pharmacopeia [21] recommended potentiometric titration method with 0.1 M NaOH for ranitidine hydrochloride.

4. Reported Methods of Analysis

4.1. Diltiazem

Literature survey revealed that a number of assay methods have been used for analysis of diltiazem in bulk drug, pharmaceutical preparations, and serum using different techniques including LC-MS/MS [2224], gas chromatography [2528], and electron capture gas chromatography [29].

4.1.1. Spectrophotometric Methods

Many research papers have described the spectrophotometric methods for the detection of diltiazem in raw materials and different pharmaceutical formulations. Recently Ayad et al. [30] established a method by kinetic spectrophotometric and spectrofluorimetric technique using NBD-Cl (4-Chloro-7-nitrobenzene-2-oxa-1,3-diazole). The reported method has many advantages in contrast with other methods in terms of sensitivity and selectivity by way of high-quality precision and accuracy. Hosny [31] reported an ion-pair complex formation method for the assaying of diltiazem in its formulation by using chromotrope 2R and rose bengal reagents. Another spectrophotometric determination of diltiazem by ternary complex formation reaction with cobalt thiocyanate at acidic pH (3–5). After extraction complex was deliberated at 627 nm [32]. Pietraś et al. [33] compare the classical and derivative spectrophotometric method for the quantitations of diltiazem in bulk and formulation. Assay was performed by the first- and second-order method with the application of “peak-zero” and “peak-peak” techniques with excellent correlation coefficient ( ). Sabino et al. [34] presented a very sensitive determination of diltiazem by the reduction of Cu(II) in buffer (pH 7) and micellar medium be full of 4,4-dicarboxy-2,2 biquinoline acid. Cu(I) produced as a result of reduction of Cu(II) reacts with 4,4-dicarboxy-2,2 biquinoline acid-formed complexes which was quantitatively detected at 558 nm. Similarly, chloro drug derivative of diltiazem was formed by means of reaction involving tertiary amino group of diltiazem and sodium hypochlorite. Newly formed derivative then reacted with starch and potassium iodide in sodium bicarbonate solution given blue color complex quantitatively read out at 540 nm [35]. Leitãoa and Esteves da Silva reported a spectrophotometric method based on kinetic study by factorial analysis. Drug was reacted in two stages with hydroxylamine and a ferric salt; as a result, hydroxamic acid and ferric hydroxamate were formed, respectively [36]. Diltiazem was quantitatively investigated in capsule and tablet dosage form by means of reaction with sodium metavanadate in acidic medium (sulfuric acid) and spectrophotometrically measured at 750 nm [37]. Ayad et al., [38] describe an analytical method by means of oxidation reaction of diltiazem with iron(III) in acidic environment while iron(III) is converted into iron(II) which reacted with the 1,10-phenanthroline or 2,2-bipyridyl in the direction of stable complexes deliberated at 510 and 520 nm, respectively. In the same way, another oxidation-reduction reaction of diltiazem was established by the El-Didamony with N-bromosuccinimide (NBS). Remaining NBS was then measured by the decline in absorbance of amaranth dye or by ceric ammonium sulfate whereas, remaining oxidant was deliberated by diminish in the shade of chromotrope 2R or rhodamine 6G [39].

A simple and reproducible method based on the formation of hydroxamic acid which reacts with iron(III) forming a complex with a maximum absorption at 525 nm [40], coloured complex of the drug with cobalt thiocyanate and methyl orange [41], showed linearity in the range 60–600 and 5–60 μg mL−1 with the absorption maxima at 630 and 420 nm, respectively. Indirect determination of diltiazem in pure form and pharmaceutical formulations presented by Kamath and Shivram [42]. Kamath also developed a selective method using the ferric hydroxamate method and detected at 500 nm in the range of 50–800 μg mL−1 [43]. Spectrophotometric method in pharmaceuticals was based on the reaction of hydrolyzed diltiazem with folin-Ciocalteu reagent and measurement of the coloured compound at 750 nm [44]. Formulations of diltiazem were also determined by extractive spectrophotometric methods using bromothymol blue, bromophenol blue, and bromocresol green [45]. Abu-Shadi et al. [46] established two accurate and selective methods that comprise the reaction of 2,3 dichloro-5,6-dicyanoquinone DDQ with dipyridamole, celiprolol, and diltiazem to develop highly coloured radical anions. In the second method, Fe(III) hydroxamate reacts with diltiazem to produce red complexes. In vitro dissolution study was performed for 240 minutes and measured spectrophotometrically at 240 nm. Parameters of mean dissolution time and time for 70% dissolution ( ) were also determined [47].

4.1.2. HPLC Methods

At present, HPLC is the most widely used technique for the analysis of bulk drugs and their formulations [48]. Derivatization of the drugs prior to analysis is normally not required. The sample preparation is extremely simple and the errors associated with it are generally kept to a minimum by using HPLC [49]. Several examinations using HPLC for determination of diltiazem (Table 2) in bulk drug substances and their formulations have been reported. Indian authors, Cumar et al., [50] documented an analytical method as per ICH guidelines for bulk and tablet dosage form on Zorbax-SB Phenyl column by using UV detector. Linearity was performed on 50–150 μg mL−1 range while, correlation of coefficient was fitted at 0.9987. Sultana et al. [51] provides a well-organized and reproducible separation of the components with UV detection. A reliable and simple method was presented with a binary solvent system. Standardized extraction procedures for drugs in various dosage forms were also developed and successfully applied to a wide range of current pharmaceutical formulations [52]. Trans-Diltiazem, 7 known, and several unknown related compounds were separated from diltiazem-HCl. Minimum detectable amount and the relative standard deviation of the assay procedure were 0.1 and 0.15%, respectively [53].

4.1.3. Bioanalytical Methods by HPLC

Several investigations performed [5456] on development of HPLC methods for determination of drugs in biological fluids. Li et al. [57] presented an HPLC method and applied in pharmacokinetic studies. Drug was extracted in mixture of hexane, chloroform, and isopropanol (60 : 40 : 5 v/v/v) and separated with isocratic mode. Method was linear over the clinical range of 0–300 ng mL−1 with 3 ng mL−1 of LOD. Spiked extraction recoveries of diltiazem were found to be 91.4–104.0%.

Kinney and Kelly reported [58] an estimation of diltiazem in plasma using normal-phase column liquid chromatography with ultraviolet detection. Chaudhary et al. [59] developed a liquid chromatographic method in human serum involving one-step deproteinization of serum by using acetonitrile. A sensitive method was developed by using an end-capped reversed-phase column with a regression coefficient of 0.9998 [60]. Several writers have carried out the determination of diltiazem in biological samples after liquid-liquid extraction by means of ethyl acetate [61], methyl tert-butyl ether [6266], a mixture of n-hexane-methylethanol-dichloromethane-mmonia [67], and hexane and isopropanol [68]. A few of these schemes engage extraction of diltiazem with organic solvents and back-extraction into phosphoric acid [64], hydrochloric acid [61, 63, 65], and sulfuric acid [62, 68]. However, these attempts are reported to be adequately sensitive, the drug has been deliberated using either a large plasma sample volume or the suggested extraction procedures give rise to poor separation of the investigated drug from endogenous interferences in biological specimens, and recovery of about 80–85%. Ascalone and Dal Bo [69] and Mangani et al. [70] have projected an automated HPLC estimation of diltiazem in human plasma by means of a column-switching technique for online cleanup. These investigations have a very low limit of detection while some drawbacks are their high reagent consumption and the need of two pumps. Hubert et al. [71] described an automated method for the quantitation of diltiazem in human plasma via liquid-solid extraction on a disposable cartridge coupled to HPLC. Christensen et al. [72] and Ascalone et al. [73] have executed the partition of diltiazem from plasma with solid-phase extraction on C18 cartridges; however, this type of cartridge gave relatively low recovery values (75–80%). These investigations overcome problems caused by endogenous compounds. Zendelovska et al. [74] propose a new method in plasma samples using solid-phase extraction on RP-select B cartridges and liquid-liquid extraction with diethyl ether. The rewards of these methods are low reagent consumption, short analysis time, ease of operation, no matrix interferences, and satisfactory limit of quantification to enable pharmacokinetic studies of diltiazem. Rustum [75] utilized a salting-out extraction procedure and polymeric reversed-phase column for the determination of diltiazem in human whole blood.

HPLC analysis of diltiazem in plasma for pharmacokinetic study in a healthy man also has been reported [76]. Pharmacokinetic parameters of diltiazem were determined using a sensitive and reproducible HPLC method which followed international standards in validation of the analytical assay [77]. Another method consists of extraction of the drugs by using hexane-isopropanol (98 : 2) followed by solvent evaporation and redissolution of the residue in MeCN-0.5 M KH2PO4 (pH 2.5, 30 : 70) [78]. Synchronized detection of diltiazem and six of its metabolites is known to occur in humans. The assay was successfully applied to detect the kinetics of these drugs in healthy volunteers [79].

4.1.4. Simultaneous Determination

Diltiazem was also simultaneously determined by HPLC technique in bulk, dosage form and successfully applied in human serum with other classes of drugs due to their coadministration. Prior to determination method was optimized and validated according to respective ICH guidelines. For the first time in the literature diltizem and statins (simvastatin, rosuvastatin and atorvastatin) were separated on isocratic eluation by RP-HPLC with UV detection. Method was optimized with different ratios of mobile phase, C18 columns, organic modifier, and pH of mobile phase. Method was found to be linear (0.625–20 mg mL−1), accurate (97.9–102%), precise ( ), and also robust with short time of analysis (<10 min) [80]. In the same way, diltiazem and lovastatin were simultaneously estimated by Kulkarni et al. [81] and, for the first the time, reported in writing style. Both the drugs were separated on Kromasil C18 column with UV detector in the presence of propranolol as an internal standard. Simultaneous determination of diltiazem and three hypoglycaemic drugs, type II (non-insulin-dependent) diabetes mellitus are reported in 2011. Method was optimized and validated isocratically mode and no significant meddlings were found from tablet excipients. Intraday (0.56 to 1.31%) and interday (0.56 to 1.39%) precision and accuracy/recovery (98–102%) were within limits. All the analytes were eluated within 6 min which is important for quality control analysis) [82]. RP-HPLC method of diltiazem with four commonly prescribed non steroidal anti-inflammatory drugs (NSAIDs) is also documented by Sultana et al., in accordance with the ICH guidelines. Separation was achieved on C18 column at ambient temperature. Method was linear in the desired range 1.25–50 μg mL−1 with the best resolution and recovery in serum (98.97–100.4%) [83]. Another simple, reproducible, and economically cost-effective HPLC method for quantitative determination diltiazem and quinolones is reported by Sultana et al., in 2009. Diltiazem and quinolones were efficiently separated on Hiber, 250-4.6 STAR RP-18 column in the presence of internal standard. The developed method was verified by later in vitro interaction studies of diltiazem with fluoroquinolones [84]. Shabana et al. describe a simple RP-HPLC method for simultaneous separation of diltiazem, prazosin hydrochloride, and two other calcium channel blockers on 10-micron particle size column. The developed method has advantages over other reported methods due to their short time of analysis (within 7 minutes) and for their sensitivity; that is, LOD and LOQ were achieved at 0.0542 and 0.1642 μg mL−1, respectively [85]. Diltiazem, cimetidine, ranitidine, and famotidine were simultaneously separated on Purospher STAR RP-18 end capped (5 μm) with isocratic eluation. Calibration curves were linear within the range of 2–12 μg mL−1 with excellent correlation coefficients ≤0.999. The developed method was competent with high precision (≤1.9%) and accuracy (96.1–102.5%) and successfully applied for the in vitro interaction studies of diltiazem and H2 antagonists [86]. However, several analytical procedures have also been described for simultaneous separation and enantio-separation of diltiazem, its analogs, possible degradation products, and metabolites. Simultaneous identification and quantification of six commonly prescribed calcium channel blocking drugs by single-step extraction method was established by Jianchun from alkaline samples [87]. The HPLC modes show adequate sensitivity and reproducibility to cover the concentration range 10–400 ng mL−1 required as therapeutic levels in plasma for these drugs [88]. Li et al. [89] developed a common method for the detection of diltiazem, verapamil, nifedipine, nitrendipine, and their active metabolites using a C18 column. The detection limit of the assay was 8 ng mL−1 for all compounds over the concentration range 1–1000 ng mL−1. The optical isomers of dL-cis-diltiazem were detected by hydrolysis of and esterification with the chiral reagent d-2-(2-naphthyl) propionyl chloride. RP HPLC separation was achieved with 0.01 M NH4OAC-MeCN (1 : 9), pH 6.6, and detection was performed at 254 nm [90]. Direct enantiomeric seperation of cis-(±)diltiazem in plasma was achieved on ovomucoid [91] and topical 2% diltiazem formulations [92] column. The former involves a deproteinization of serum and acetonitrile used for sample cleanup. The degree of extraction of the enantiomers was 86% in the later method. Packed column subcritical and supercritical fluid chromatography (p-sub or pSFC) has been used as a powerful chiral separation technique, for the resolution of optical isomers of diltiazem. P-sub- or p-SFC tends to obtain higher column efficiency than normal-phase HPLC [93]. Direct resolution of four optical isomers was completely resolved on both normal- and reversed-phase chiralcel of columns. This chiral resolution was applied to determine 3 optical impurities that might be present in diltiazem and its tablets [94]. The enantiomers separations, the 8-chloro derivative of diltiazem (clentiazem maleate) and their deacetyl forms were resolved on ovomucoid-bonded chiral stationary phase with a mobile phase of acetonitrile-0.02 M phosphate buffer (pH 6.0) [95]. S(-)N-1-(2-naphthylsulfonyl)-2-pyrrolidinecarbonyl chloride (a highly optically pure derivatization reagent) was developed for converting enantiomers into diastereomers for subsequent resolution by high-performance liquid chromatography [96]. HPLC measurements of diltiazem and its metabolites were also investigated in biological fluids [97104]. Diltiazem was detected in the presence of desacetyldiltiazem on Lichrosorb RP 18 column with acetate-MeCN-MeOH (50 : 40 : 10) and also purified by preparative HPLC on LiChroprep Si60 with petroleum ether-EtOAc-MeOH (50 : 50 : 5) as mobile phase [105]. Simultaneous analysis of diltiazem, N-monodemethyldiltiazem, and desactyldiltiazem were identified with the extraction efficiency of 95–100% and the limit of detection was 5 ng mL−1 [106]. Different C18 columns are compared with the influences of organic cosolvent, pH, ionic strength and the addition of various amines for the detection of diltiazem and its 5 metabolites [107]. Goebel and Koelle described a very sensitive method for the determination of diltiazem and four of its metabolites in plasma samples. Calibration curves were linear between 1 and 800 ng mL−1 and the lower limit of detection was 0.1-0.2 ng mL−1 [108]. HPLC combined with mass spectrometry was used for the analysis of diltiazem and its 3 metabolites in blood plasma of patients treated for ischemic heart disease [109]. A sensitive and automatic method for the analysis of diltiazem and its main metabolites in acidified serum is described using solid-liquid extraction on disposable extraction cartridges. This method avoids the in vitro degradation and eliminates the numerous manipulations involved in liquid-liquid extraction [110]. Tracqui et al. [111] developed an automated method by using liquid-solid extraction on disposable extraction cartridges in combination with HPLC. Lee et al. established [112] a method that involves the use of two 5 μm BDS silica gel columns 15 cm (4.6 mm I.D) in series for increased resolution and sensitivity and organic mobile phase for both extraction and elution of diltiazem. Dasandi and his coworkers [113] have described a high-throughput and specific method using ultraperformance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). Compared with the published methods, the narrow peaks produced by UPLC are of particular advantage when coupled to electrospray mass spectrometry and offering superior sensitivity. Total run time was 2.0 min only; therefore, the method is particularly suitable for routine assay. Zoest and his companions [114] developed a sensitive, specific, and reproducible assay for the pharmacokinetic study of diltiazem, primary and secondary metabolites of diltiazem.

4.2. Cimetidine
4.2.1. Spectrophotometric Methods

Sensitive spectrophotometric method based on the reaction of CIM, RAN, FAM, and NIZ with N-bromosuccinimide (NBS) and subsequent measurement of the excess NBS by its reaction with p-aminophenol to give a violet-colored product. Decrease in the absorption intensity ( ) of the colored product, due to the presence of the drug, was correlated with its concentration in the sample solution ( at 552 nm) [115]. Another sensitive method presented by Darwish et al., [116] for determination of H2 receptor antagonists based on the oxidation of these drugs with cerium(IV) in presence of perchloric acid and subsequent measurement of the excess Ce(IV) by its reaction with p-dimethylaminobenzaldehyde to give a red-colored product ( at 464 nm). Fast spectrophotometric methods for the analysis of CIM in pharmaceuticals and human urine using batch and flow-injection procedures based on the formation of a green complex drug and Cu(II) in acetic/acetate medium, measured at 330 nm [117]. Two fast methods developed by Garcia et al. [118] using batch and flow-injection procedures for CIM based on the formation of a green complex with Cu(II) in acetic/acetate medium, measured at 330 nm. Kelani et al. [119] described three sensitive stability indicating spectrophotometric methods using sodium nitro prusside, 3-methyl-2-benzo-thiazolinone hydrazine, and cobalt(II), respectively.

4.2.2. HPLC Methods

CIM is highly polar drug; therefore, several RP-HPLC methods reported on the analysis in pure form, tablet and biological fluids (Table 3) using either a C8 [120], C18 [121127], or polymeric column [128]. Determination of CIM in human plasma and urine samples has been investigated by the solid-phase or liquid-phase extraction techniques [121124, 129143] and extrelut extraction [144]. These methods have some drawbacks, including the need of large volume plasma extraction [133135], poor recovery [136, 137], utilization of an internal standard that is not commercially available [135, 138], requirement of an additional protonation step [139], low capabilities of urine analysis [133, 139, 140], and triple extraction [142, 143] which were time consuming. HPLC method presented by Jantratid et al. [145] for the estimation of CIM in human plasma involves a simplified sample preparation by protein precipitation with perchloric acid.

Iqbal et al. [146] reported an HPLC method for CIM in biological fluids. Plasma sample preparation needs liquid extraction with water-saturated ethyl acetate. After extraction, extracts were evaporated under nitrogen and reconstituted with mobile phase. Advantages of this validated assay over other reported methods include a simple plasma extraction and urine dilution procedures and willingly available internal standard and applicability to clinical studies. Sample cleanup and analysis of CIM in urine and blood were described by Apffel and his friends [147], sample preparations were achieved by adsorption technique on Waters Sep-pak C18 disposable precolumns, and the separation was performed on a RP column. Dong-Sun et al. [148] presented an analytical work saving online-column switching HPLC method with UV detection for cleanup and analysis. Gomita et al. [149] established an HPLC method for microdetermination of CIM in rat plasma. Betto et al. [121] developed an HPLC method with diode-array detector in order to assay CIM and its related impurities in pharmaceutical formulations.

4.3. Ranitidine
4.3.1. Spectrophotometric Methods

Three spectrophotometric methods for the determination of RAN involve formation and solving of simultaneous equations, making use of first-order derivative spectroscopy and two wavelength calculations presented by Pillai and Singhvi [150]. Kinetic spectrophotometric method involves the reaction of the drugs with alkaline potassium permanganate, and green color is produced peaking at 610 nm. The reaction is monitored by measuring the rate of change of absorbance of the resulting manganate species [151]. Another kinetic method described by Walash et al. [152] for RAN and NIZ based on the reaction of the compounds with 7-chloro-4-nitrobenz-2-oxa-1,3-diazole in borate buffer (pH 7.4) at 60°C for 25 min and the reaction product is measured at 495 nm. The protolytic constants of RAN and NIZ were studied using a spectrophotometric method [153]. Four new methods established by Basavaiah and Nagegowda [154] using titrimetry and visible spectrophotometry with potassium bromate as the oxidimetric reagent and acid dyes, methyl orange, indigo carmine, and metanil yellow. Determination of RAN and NIZ through charge transfer complex formation with either p-chloranilic acid or 2,3 dichloro-5,6-dicyanoquinone (DDQ) and the colored products are quantified at 515 and 467 nm in chloranilic acid and DDQ methods, respectively [155]. Charge-transfer complexation method for RAN, FAM, and NIZ is based on interaction of these drugs with 7,7,8,8-tetracyanoquinodimethane to give green-colored radical anions that are deliberate at 840 nm [156].

4.3.2. HPLC Methods

Literature showed that a variety of HPLC methods have been reported for the estimation of RAN (Table 3) in biological fluids by using a short polymeric chain [157], silica-based column [158], polymeric base reversed-phase column [159], without solvent extraction or one-step sample preparation [158, 160162], two-step extraction [163], two solvents combination for extraction [164], protein precipitation and salting out procedures [157], solid-phase extraction [165167], liquid-liquid extraction with organic solvents [168170], and ion-pair high-performance liquid chromatography [161, 163]. Farthing et al., [171] reported a HPLC method for RAN in human plasma by solid-phase extraction and midbore chromatographic technique.

Prueksaritanont et al. [172], Rahman et al., [173], and Viñas et al. [174] established sensitive methods for the concurrent determination of RAN and its metabolites (RAN N-oxide, RAN S-oxide, and desmethylranitidine) in human plasma and urine. Lant et al. [175] developed an assay for RAN and all the above metabolites in biological fluids by a postcolumn fluorimetric derivatization. Quantitative as well as qualitative analysis of RAN and its metabolites were determined by a direct liquid introduction-RP HPLC-mass spectrometry system [176].

Kokoletsi and his companions [176] presented a method for the simultaneous determination of RAN, methylparaben, and propylparaben in oral liquids. Purifications of samples were carried out by solid-phase extraction using a copolymeric sorbent.

4.4. Famotidine
4.4.1. Spectrophotometric Methods

Amin and his companions [177] reported three selective spectrophotometric methods for the determination of FAM through oxidation with n-bromosuccinimide and ceric sulphate by measuring the absorbance at 521 and (528, 526) nm, respectively. Dissolution test for FAM tablets was optimized using a simple direct spectrophotometric flow injection assay ( 265 nm) that has also been fully validated in terms of linearity, limit of detection, precision, selectivity, and accuracy [178]. Different UV spectrophotometric methods have been reported for the determination of FAM in pharmaceutical formulations based on the reaction of FAM with 1,4-benzoquinone [179] and sodium nitroprusside [180], interaction of FAM with ninhydrin [181] and p-chloranilic acid [182], charge transfer complexation with chloranil, 2,3-dichloro 5,6-dicyano 1,4-benzoquinone and dichloronitrophenol [183], measuring the peak height of the second derivative [184], ion-pair complex formation with bromocresol green and bromothymol blue [185, 186], and difference spectrophotometric and quadratic polynomial coefficient methods [187].

4.4.2. HPLC Methods

Literature survey showed that a number of HPLC methods have been reported for the individual and simultaneous determination of FMT [188192] and of all related compounds (Table 3) [193199]. Majority of methods utilized either solid phase or liquid-liquid extraction techniques [200203]. Extensive methods were reported [204, 205] for FAM in which HPLC separation with column-switching technique was applied. RP-HPLC method for the determination of FAM and its potential impurities in dosage forms were performed on a Supelcosil C18 column with an isocratic mobile phase. Response was linearly dependent on concentration 1–80 μg mL−1 (regression coefficient, from 0.9981–0.9999) [206]. Carlucci et al. [207] established an RP-HPLC and second-derivative ultraviolet spectra in finished product by utilizing the linear relationship between concentration and derivative peak amplitude. Campanero et al. [190] presented a liquid chromatography-electrospray ionization mass spectrometric detection with a rapid and well-organized sample refine method for the estimation of FAM in human plasma. Assay method using a monolithic column has been developed for quantification of FAM in plasma and enables the measurement of FAM for therapeutic drug monitoring with a minimum detectable limit of 5 ng mL−1 [208].

Stability-indicative liquid chromatographic analysis and degradation kinetics of FAM in tablets are described by Kamath and friends [209]. Another stability-indicating assay was demonstrated for the simultaneous determination of FAM and related impurities in pharmaceuticals using porous graphitic carbon column [210].

4.5. Simultaneous Determination of H2 Receptor Antagonists

Literature shows several articles published for the simultaneous determination of H2 receptor antagonists (cimitedine, ranitidine, and famotidine) with other categories of drug classes (Table 4). An HPLC method on fourth-generation fluoroquinolones that is, gemifloxacin and cimetidine, ranitidine and famotidine was established the presence of getifloxacin as an internal standard by the Pakistani authors. Mobile phase was delivered isocratically at ambient temperature. Quantitation was performed at three different wavelengths, that is, 221, 256, and 267 nm as well as statistical analysis such that Student’s -test and Friedman’s were also functional to compare these outcomes [211]. Naveed et al. [212] presented a RP-HPLC method for the simultaneous determination of lisinopril and all three H2 receptor antagonists. Separation was performed isocratically with 1.0 mL min−1 flow rate over the concentration range of 2.5–100 and 0.7–12.50 μg mL−1 for H2 receptor antagonists and lisinopril, respectively, with regression value of ±0.999. In the same way captopril, a specific ACE inhibitor, was quantitatively determined by the RP-HPLC technique with CIM, RAN, and FAM. For method optimization different mixtures of mobile phase were tried and retention time, resolution, and peak symmetry were also taken into consideration for system suitability. Later on the validated method was efficiently verified for in vitro interaction studies [213]. Arayne et al. [214] also developed and validated a high-performance liquid chromatographic method for metformin, ranitidine, famotidine, and cimetidine in their synthetic mixtures and dosage forms. Calibration curve was constructed in the concentration range of 5–25 μg mL−1 with 0.997–0.9998 values. Inter-and intraday precision and accuracy were less than 2% and 98.6–102.4%, respectively. Another sensitive and selective liquid chromatographic method was published for the prazosin, CIM, RAN, and FAM by Najma et al. [215] as per (ICH) guidelines. Mobile phase was pumped in column through isocratic mode. Total run time of analysis was less than five minutes that is important for quality control analysis and clinical laboratories. Similarly, an HPLC method for the simultaneous estimation of a fluoroquinolone sparfloxacin and above H2 receptor antagonists was also reported in bulk, dosage form, and human serum. The demonstrated method was specific as there is no interference found from the spike matrix and serum. Accuracy and precision of the method were found in acceptable boundary 98.02–102.136% and 0.5–2.2%, respectively. Statistical Paired -test was applied for the verification of the collected data [216]. HPLC simultaneous determination of CIM, RAN, FAM, and NIZ was established using a two-level, full-factorial design with three variables (volume of methanol, percentage of triethylamine, and concentration of phosphate buffer). None of the commercial samples was found to be outside the compendial limits of 90–110.0% of the claim amount [217]. HPLC method for CIM and RAN in plasma sample was investigated by cleanup procedure that used solid-phase extraction and separated on a Lichrocart Lichrospher 60 RP-select B column. The effects of organic modifiers, mobile phase composition, pH, and buffer concentration on retention time of analytes were also investigated [218].

5. Conclusion

In this paper UV and HPLC methods for the determination of diltiazem and three H2-receptor antagonists in bulk material, pharmaceutical formulations and biological specimens are reviewed. Spectrophotometric techniques provided practical (less-time-consuming, simpler, and more convenient) and significant economic advantages over other methods; therefore, they are a frequent choice for pharmaceutical analyses. HPLC methods generally required complex and expensive equipment, provision for use and disposal of solvents, labor-intensive sample preparation procedure, and personal skills in chromatographic techniques. In addition, most of the HPLC methods reviewed have the potential application to clinical research of drug combination, multidrug pharmacokinetics, and interactions.