The API is supplied as 1-Phenyl-1H-tetrazole-5-thiol, synonym 1-Phenyl-5-mercaptotetrazole, with the product descriptor Pharma Grade API for Tablet / Capsule / Granule / Injection, Oral & Injectable. The molecular formula is C7H6N4S, the relative molecular mass is 178.21 g/mol, and the CAS registry number is 86-93-1. At release the product is a white to off-white crystalline powder with a melting range of 143–146°C determined by USP <741>. The substance is a heterocyclic thiol-thione tautomer, and the position of the tautomeric proton can vary with solvent polarity, pH, and temperature. This variability influences the analytical control strategy because the identity test and related-substances method must be capable of detecting the thiol and thione forms as well as the oxidative disulfide dimer.

The product model/grade descriptor is the full supplier designation 1-Phenyl-1H-tetrazole-5-thiol; 1-Phenyl-5-mercaptotetrazole Pharma Grade API for Tablet / Capsule / Granule / Injection, Oral & Injectable. No USP or Ph. Eur. monograph is generally available for this substance. The release specification is therefore built on ICH Q6A principles and uses in-house HPLC methods for assay and related substances. Synthesis of the tetrazole ring often involves azide chemistry, so residual azide, where relevant, is controlled separately; technical-grade material may not have this control. Manufacturing of the pharma-grade material is conducted under ICH Q7 and EU GMP Part II, with batch traceability, change control, and stability data.

Typical synthetic use is introduction of the 1-phenyl-1H-tetrazol-5-ylthio group into advanced pharmaceutical intermediates through thioether formation on activated alkyl halides, mesylates, or tosylates. The free thiol is sparingly soluble in water, so in granulation and capsule filling it is usually dispersed as a milled solid rather than dissolved. A solid oral particle-size target of D90 ≤20 µm is commonly used to support USP <905> content uniformity in low-dose blends; this is a formulation-derived target, not an intrinsic pharmacopoeial limit.

What Release Limits Apply to This 1-Phenyl-5-mercaptotetrazole Across Oral and Parenteral Routes?

The release specification for the pharma-grade material includes assay, related substances, residual solvents, elemental impurities, loss on drying, residue on ignition, melting range, particle size, microbial enumeration, and bacterial endotoxins for injectable grade. Acceptance criteria are route-dependent. The oral grade is controlled for total aerobic microbial count and specified organisms per USP <61>/<62>; the injectable grade additionally requires bacterial endotoxin testing per USP <85> and a lower bioburden limit before aseptic processing. The table lists the core release criteria.

For residual solvents, the certificate of analysis reports only solvents that can originate from the final recrystallization and drying. When acetone is used as the final crystallization solvent, the ICH Q3C limit is ≤5000 ppm; for dichloromethane the limit is ≤600 ppm; for benzene, a Class 1 solvent, the limit is ≤2 ppm. These are measured by headspace gas chromatography with flame-ionization detection and, where required, electron-capture detection. If the synthetic route uses sodium azide, residual azide is controlled by ion chromatography to a limit agreed with the supplier; because azide is toxic and can form explosive metal azides, this is one of the main differences between pharma-grade and technical-grade material.

For elemental impurities, the values in ICH Q3D are not concentration limits but permitted daily exposures. They are converted to concentration limits from the maximum daily dose of the finished product. The API is screened by USP <232>/<233> for Class 1 and Class 2A elements as a minimum, and for any catalyst element intentionally used in synthesis. If the synthesis uses palladium-catalyzed coupling, a palladium limit such as ≤10 ppm is often applied; this is a supplier-specific limit because no official limit is established solely for this API.

Because no official monograph exists, the HPLC methods are validated under ICH Q2(R1) for specificity, linearity, accuracy, repeatability, intermediate precision, and quantitation limit. The related-substances method should use a C18 column with a low-pH mobile phase to suppress thiolate formation; otherwise the free thiolate can produce tailing and poor resolution from the disulfide oxidation product. The disulfide dimer is the primary degradation marker and should be monitored at a relative retention time established with a reference standard. Identification by IR should be interpreted carefully because the thiol S–H stretch may be weak or absent in the thione tautomer; the fingerprint region and current reference spectrum are more reliable than a single S–H band.

When Direct Compression Replaces Wet Granulation in Low-Dose Tablet Manufacture

Low-dose solid oral formulations of 1-phenyl-1H-tetrazole-5-thiol can be prepared by direct compression if the API is milled and dispersed onto high-surface-area excipients. Because air-jet milling can increase surface free energy and cohesion, the milled API is usually first blended with microcrystalline cellulose and lactose monohydrate for a duration derived from blend-sampling data. Blend uniformity testing per USP <905> is used to establish the mixing time; over-blending can cause segregation of a cohesive low-solubility API, so the window is not fixed.

For high-shear wet granulation, the API is not dissolved in the granulating fluid because its aqueous solubility is low. It is dry-mixed with the filler and binder, then granulated with purified water or a low-viscosity povidone K30 solution at 2–5% w/w. The granulation endpoint and drying step are the main process risks. If an aqueous granulating fluid above pH 7.5 is held in contact with the thiol for more than 30 min, oxidative dimerization may accelerate; published data for this specific API under alkaline granulation is limited, so the pH and hold time should be confirmed by process development. A conservative drying endpoint of loss on drying ≤1.0% w/w is usually selected to suppress moisture-promoted degradation.

Drying on a fluid-bed dryer with inlet air temperature set at 50°C and product temperature not exceeding 35°C is a conservative starting point. Product moisture is checked at intervals after the first 20 min; the exact interval is equipment-dependent. If the granules exceed 2.0% w/w moisture for prolonged periods, related-substance increases may be observed, but this must be verified for the specific batch size and dryer load.

For capsule filling or granule dosing, the dried granules are milled through a 0.8–1.0 mm screen and filled into hard gelatin or hydroxypropyl methylcellulose capsules. The granule fill weight is adjusted using tapped density and flow measurements; if the milled API is cohesive, roller compaction may be used to produce denser granules before capsule filling. Tablet film coating should be preceded by a seal coat if aqueous coating is used; moisture uptake through the tablet surface can mobilize the thiol and produce disulfide-related discoloration. Coating pan bed temperature of 40–50°C is a typical starting range, and the coat weight is minimized to reduce moisture load.

For oral tablet and capsule development, dissolution testing per USP <711> is required to establish the release profile. Because the API has low water solubility, the dissolution medium may require a surfactant such as sodium lauryl sulfate at 0.1–1.0% w/w to achieve sink conditions; the exact surfactant concentration must be justified from solubility data. The BCS classification of this compound has not been assigned in public regulatory lists; its low water solubility suggests a BCS II or IV profile depending on intestinal permeability, but published data for this specific API is limited.

Injectable Solubilisation and Terminal Sterilizing Filtration Boundaries

The injectable grade requires low bioburden and endotoxin before aseptic processing. The compound is typically dissolved by pH adjustment with sodium hydroxide or tromethamine in nitrogen-sparged Water for Injection. Dissolved oxygen should be reduced below 0.2 mg/L and redox-active trace ions such as iron and copper should be controlled, because aromatic thiols can undergo metal-catalyzed oxidation to disulfides. Edetate disodium at 0.01–0.05% w/w may be added to chelate free metal ions; the exact concentration is formulation-dependent and must be justified by stability data.

Terminal sterilizing filtration through a 0.22 µm polyethersulfone or polyvinylidene fluoride membrane is used where the formulation is chemically compatible. Filter compatibility studies should include adsorption, flux decay, and extractables assessment per USP <1663>/<1664>. Steam sterilization is not recommended unless the formulation is shown to resist disulfide formation at 121°C for 15 min; published data for this specific API under steam sterilization is limited, so aseptic filtration is generally preferred for oxygen-sensitive thiol formulations.

The equilibrium solubility of this compound in water is low; solubility data should be generated at relevant pH values using Ph. Eur. 5.11.1 or a shake-flask method. For injection, pH adjustment is used to form the more soluble thiolate; however, the thiolate is more oxidation-prone than the neutral thiol, so solubilized solutions should be filled immediately or held under nitrogen with a defined hold time. For injectable products, subvisible particle counts are controlled using USP <788> or Ph. Eur. 2.9.19; the API itself is not a finished injectable solution, but filter compatibility and particle burden must be assessed before final formulation.

Differences From 1-Methyl-1H-tetrazole-5-thiol and Technical-Grade 1-Phenyl-5-mercaptotetrazole

Structural substitution at N-1 separates this product from 1-methyl-1H-tetrazole-5-thiol. The phenyl derivative has a relative molecular mass of 178.21 g/mol, compared with 116.14 g/mol for the methyl analog. The added phenyl ring increases lipophilicity and reduces water solubility, shifting reverse-phase HPLC retention and modifying extraction behavior in liquid formulations. The methyl analog is the more familiar 3-position side-chain unit in several cephalosporin APIs, whereas the phenyl derivative is selected when an aromatic ring is required for electronic or steric reasons. No pharmacopoeial monograph is generally available for the phenyl analog; the supplier-driven specification therefore carries greater weight in regulatory review.

Chromatographically, the phenyl compound retains longer than the methyl analog under reversed-phase conditions because of increased hydrophobic surface area. This retention difference is used to confirm that the correct tetrazolethiol has been charged; a mixed retention time or co-elution with the methyl analog is a batch-record discrepancy that should trigger an investigation. Compared with 5-mercapto-1H-tetrazole, the N-1 phenyl substituent replaces a proton at the tetrazole nitrogen and alters the thiol-thione equilibrium. The heteroaromatic tetrazole ring stabilizes the thiolate anion by resonance, lowering the pKa relative to aliphatic thiols and reducing free-thiol odor; this property is relevant in solid oral formulations where odor control and oxidative stability are part of the target product profile.

Technical-grade 1-phenyl-5-mercaptotetrazole differs in residual solvent profile, elemental impurities, microbial control, particle-size capability, and documentation. Technical-grade material is often sold for photographic antifogging, metal complexation, or chemical intermediate use and may contain non-volatile impurities that are not acceptable by oral or injectable standards. Pharma-grade material is manufactured under ICH Q7 and EU GMP Part II, with release testing for residual solvents and elemental impurities that technical-grade material may not provide. For injectable use, endotoxin and subvisible particle control are additional differentiators; technical-grade material is therefore unsuitable for parenteral applications.

For tablets and capsules, process validation should cover the particle-size distribution of the milled API, blend uniformity, granule moisture, and compression force. For injectables, the validation should include aseptic process simulation, filter compatibility, and container-closure integrity. These activities follow 21 CFR 210 and 21 CFR 211 for finished dosage forms or equivalent GMP requirements in the target region. The API should be stored in tightly closed, light-resistant containers under nitrogen at 2–8°C; long-term storage under ambient humidity is not recommended without moisture barrier packaging. A retest date is assigned from ongoing stability data under ICH Q1A and is stated on the certificate of analysis.