Aciclovir Eye Drops Veterinary Grade API is supplied as the synthetic purine nucleoside 9-[(2-hydroxyethoxy)methyl]guanine, CAS 59277-89-3, with molecular formula C8H11N5O3 and molecular mass 225.21 g/mol. The material appears as a white to almost white crystalline powder. The free base has an aqueous solubility of approximately 1.6 mg/mL at 25 °C and pKa values of 2.27 and 9.25; this solubility profile is the principal constraint in solution and parenteral formulation and is the reason ophthalmic products are prepared as suspensions. Two particle-size variants are released: ACV-V-API-N for tablets, capsules, powders, granules, and premix, and ACV-V-API-M for ocular suspensions, with the micronized variant controlled to D90 ≤ 15 µm and D50 ≤ 5 µm by laser diffraction per ISO 13320-1. Manufacturing is performed under EU GMP Part II for active substances. The veterinary-grade designation indicates that lot release includes TSE/BSE documentation, bacterial endotoxin control for ocular and injectable routes, and residual solvent verification against VICH GL18 and Ph. Eur. 5.4. Aciclovir is activated by viral thymidine kinase to the monophosphate and subsequently by cellular kinases to aciclovir triphosphate, which inhibits herpesviral DNA polymerase; this activation mechanism limits its antiviral spectrum to herpesviruses expressing the relevant kinase and differentiates it from host-polymerase-directed antiviral agents.

The pharmacopoeial release grid for aciclovir and its veterinary-specific extensions

Release testing uses Ph. Eur. monograph 0968 and the USP Aciclovir monograph where applicable. Identification is confirmed by infrared absorption spectrophotometry Ph. Eur. 2.2.24 and by HPLC retention time against aciclovir CRS Ph. Eur. 2.2.29. Assay on the dried basis is controlled within 99.0%–101.0%. Related substances are determined by HPLC with limits for guanine at ≤ 0.1%, any unspecified impurity at ≤ 0.10%, and total impurities at ≤ 0.5%. Residual solvents by headspace GC follow VICH GL18 and Ph. Eur. 5.4; methanol is limited to ≤ 3000 ppm, dichloromethane to ≤ 600 ppm, and unlisted solvents below 10 ppm unless process development data justify an alternative limit. Elemental impurities are managed under ICH Q3D with the USP <232>/<233> platform or Ph. Eur. 2.4.20. Loss on drying by Ph. Eur. 2.2.32 is ≤ 0.5%, and sulfated ash by Ph. Eur. 2.4.14 is ≤ 0.1%. The micronized ocular grade is characterised for bulk density and specific surface area because these parameters influence dispersibility and preservative adsorption when the API is transferred into ophthalmic vehicles. Storage in double polyethylene-lined fibre drums at ≤ 25 °C is specified; open handling above 60% RH requires resealing or nitrogen purging to prevent agglomeration and surface hydrolysis.

Why Is Resuspendability in Ophthalmic Suspension a Function of Particle Size and Vehicle Ionic Strength?

A 0.3% w/v aciclovir ophthalmic suspension using the free base will contain only about 1.6 mg/mL in true solution at 25 °C; the remainder is suspended and must be uniformly redispersible by shaking. The micronized API is therefore dispersed in a sterile vehicle based on hydroxypropyl methylcellulose 0.5% w/v or carbomer 974P, using a rotor–stator homogenizer at 3000–5000 rpm for 15–30 min. Particle size is not a single-point criterion: D50 ≤ 5 µm and D90 ≤ 15 µm reduce corneal irritation, but excessive size reduction below 1 µm increases surface energy and may enhance aggregation in vehicles containing ionised excipients. Benzalkonium chloride at 0.005%–0.01% w/v is added as a preservative in multi-dose containers; however, carbomer vehicles can reduce the free concentration of benzalkonium chloride through electrostatic interaction, so preservative efficacy must be verified by USP <51> rather than assumed. Tonicity is adjusted with sodium chloride or mannitol to 280–320 mOsm/kg. Phosphate buffers should be avoided in carbomer-containing eye drops because gelation can occur. The suspension is normally sterilised by aseptic filtration of the vehicle and gamma-irradiated or steam-sterilised API, since terminal autoclaving of the finished suspension may accelerate particle growth and change sedimentation volume. Published data for veterinary-specific benzalkonium chloride–carbomer interaction at aciclovir loading above 0.5% w/v is limited, so accelerated sedimentation and preservative challenge should be measured at the intended final concentration.

Transfer onto oral solid-dosage lines typically starts with dry granulation because the micronized API has poor flow and low bulk density, while direct compression is usually limited to blends with high-capacity diluents and lubricant levels below 0.5% w/w magnesium stearate. A standard compacted blend contains aciclovir, microcrystalline cellulose, croscarmellose sodium, and povidone K30; roller compaction at roll pressure 30–50 bar with 1.5 mm oscillating mill screen produces granules for tablet compression and capsule filling. For tablet compression, target hardness is 60–90 N and friability is maintained below 1%; disintegration is monitored in water at 37 °C using Ph. Eur. 2.9.1. Dissolution testing is conducted with USP <711> Apparatus 2; because aciclovir has low solubility in neutral media, the dissolution medium is typically acidified or contains surfactant to maintain sink conditions. For capsules, the granules are blended with lactose monohydrate and sodium starch glycolate, then filled on a dosator machine with fill weight variation ≤ ±4% and content uniformity per Ph. Eur. 2.9.40. Powders, granules, and premix forms require carrier particle size matched to the API to prevent segregation; blending in a V-blender at 60% fill volume for 10–15 min is followed by sampling at 10 locations and HPLC assay within 90%–110% of label claim with RSD ≤ 5%. When the formulation is exposed to ambient relative humidity above 60%, agglomerates rich in aciclovir can form in transfer hoppers and cause content uniformity drift; this has been observed on production-scale lines as top-to-bottom assay variance. For granules, hydroxypropyl cellulose binder at 2%–4% w/w is used, and the wet mass is dried in a fluid-bed dryer at inlet air temperature 55–65 °C until loss on drying reaches 1.0%–2.0%.

When Aciclovir Base Is Converted to Sodium Salt for Injectable Solutions

Injectable dosage forms do not use the free base directly because 1.6 mg/mL solubility is inadequate for parenteral dosing. Instead, aciclovir sodium salt is prepared by reaction with sodium hydroxide, or the free base is dissolved in alkaline solution during compounding. The resulting solution has pH 10.5–11.5 to maintain the ionised form and prevent precipitation. This alkalinity imposes material compatibility limits: borosilicate glass or polypropylene containers are preferred, and elastomeric closures must be extracted and assessed for leachables after exposure to the alkaline solution. The solution is adjusted to physiological tonicity with sodium chloride or dextrose. Terminal sterilisation by autoclaving at 121 °C for 15 min is acceptable, but carbon dioxide ingress during cooling can lower pH and cause crystallisation; nitrogen overlay is therefore maintained through the filling line. For infusion, dilution in 0.9% w/v sodium chloride or 5% w/v dextrose is performed at time of use, and the diluted solution is used within 12 h at controlled room temperature. The main degradation pathway is hydrolysis of the 2-hydroxyethoxymethyl side chain to guanine; degradation is accelerated at pH 4–6 and by prolonged exposure to light, so the formulated solution is protected from light and stored at ≤ 25 °C. Compatibility with other veterinary injectable products should be confirmed by HPLC at 0, 6, and 12 h; published data for admixtures of aciclovir sodium with some veterinary electrolytes is limited.

Impurity, endotoxin, and species-specific regulatory distinction from human-grade aciclovir

The chemical identity of veterinary-grade aciclovir is the same as human-grade aciclovir; the distinction lies in release documentation, risk assessment, and target-species suitability. Veterinary-grade material is released under EU GMP Part II and VICH-aligned documentation, with TSE/BSE risk assessment per Ph. Eur. 5.2.8 and residual solvent control per VICH GL18. Bacterial endotoxin control is applied by the intended route: ophthalmic and injectable products use Ph. Eur. 2.6.14 with limits appropriate to the dose and body weight, whereas oral powders may not require routine endotoxin testing unless the product monograph specifies it. Elemental impurities are assessed under ICH Q3D rather than the older heavy metals limit. The material should not be regarded as interchangeable with valaciclovir, which is a prodrug with different aqueous solubility and higher oral bioavailability but is not equivalent in ophthalmic suspension behaviour. Compared with ganciclovir, aciclovir has a narrower spectrum because it requires viral thymidine kinase for first phosphorylation; published efficacy data for feline and equine herpesvirus are limited and should be supplemented with susceptibility testing where available. The API is incompatible with strong oxidising agents and acidic vehicles, because protonation below pH 9 precipitates the free base.