Oxytetracycline (CAS 79-57-2; molecular formula C₂₂H₂₄N₂O₉; relative molecular mass 460.43 g/mol as free base) is supplied as veterinary-grade active pharmaceutical ingredient in oxytetracycline dihydrate and oxytetracycline hydrochloride forms. The historical proprietary name Terramycin refers to the original fermentation-derived product, but current compendial veterinary grades are specified by salt form, assay, impurity profile, solvent burden, and endotoxin limit rather than brand name. Model designations are supplier-specific and commonly encode the salt and nominal assay, for example OTC-HCl-98 or OTC-DIH-95; particle-size classes may be appended for premix or injectable suspension grades. Because no harmonized model code exists across manufacturers, the purchase specification must state salt form, assay method, water content, related substances, residual solvents, particle-size D90, and bacterial endotoxin limit for the intended route.

The compound reversibly binds the 30S ribosomal subunit and blocks aminoacyl-tRNA acceptor-site attachment, producing bacteriostatic activity against susceptible Mycoplasma spp., Rickettsia spp., Chlamydia spp., Ehrlichia spp., and selected Gram-positive and Gram-negative bacteria. Finished veterinary dosage forms include tablets, capsules, injectable solutions and suspensions, oral powders, granules, feed premixes, and drinking-water solutions. Salt selection is process-driven: the hydrochloride is used for aqueous injectable concentrates and oral solutions requiring high solubility, while the dihydrate is used for dry blends, premixes, and granulated products where lower hygroscopicity and slower dissolution are acceptable.

Compendial release specifications are based on current USP and Ph. Eur. monographs. High-performance liquid chromatography with ultraviolet detection near 353 nm is used for assay and related-substance quantification. Typical release acceptance limits for veterinary-grade material include assay between 95.0% and 102.0% on the dried or anhydrous basis, pH of a 1.0% aqueous preparation between 2.3 and 2.9 for the hydrochloride or between 4.5 and 7.5 for the dihydrate, and total specified impurities not more than 3.0%. Hydrochloride loss on drying is normally not more than 2.0%, whereas dihydrate water content is controlled within the crystalline hydration range of 6.0% to 8.0%. Injectable grades require additional bacterial endotoxin control because the API is heat-labile and terminal sterilization of finished oxytetracycline solutions may increase 4-epioxytetracycline and anhydrotetracycline degradation products; aseptic processing with pre-sterilized or depyrogenated raw material is used.

Hard capsule filling with oxytetracycline dihydrate requires attention to powder flow because the API has a cohesive crystalline habit and may over-lubricate if magnesium stearate is used above 2.0% w/w. Capsule formulations using a dosator-type machine benefit from a precompacted slug or a granulated intermediate; powder bed depth and dosator compression ratio are adjusted to maintain net fill weight variation below 3.0%. Oral granules and powders are typically wet-granulated with 5% povidone K30 solution and dried at inlet air temperature below 50°C to final moisture of 1.0–2.0%; oversize material above 1.0 mm is rejected, and fine material below 150 µm is controlled to prevent dusting. Water-soluble oral powders for drinking-water administration are packaged in aluminum-laminated sachets with desiccant because the hydrochloride is hygroscopic and will cake at humidity above 65% RH.

Wet Granulation, Dry Compaction, and Premix Blend Uniformity

Oxytetracycline dihydrate powder is cohesive and static-prone. On rotary tablet presses operating above 40 rpm, die fill variation can exceed 5.0% unless a force feeder or glidant such as colloidal silicon dioxide at 0.5–1.0% w/w is used. Direct compression tablets for companion animals commonly add 20–40% microcrystalline cellulose and 1.0–2.0% magnesium stearate; ejection force and hardness are monitored because the API may laminate at low moisture and high compression speed. Wet granulation is confined to aqueous binders with inlet drying air below 50°C, because stereochemical degradation at C-4 accelerates under heat and alkaline or strongly acidic conditions. Final granule moisture is commonly targeted at 1.0–2.0% to reduce color change and 4-epioxytetracycline formation. Finished tablets are tested for content uniformity according to USP <905> and dissolution according to USP <711>; immediate-release formulations are typically tested in 0.1 N hydrochloric acid because dissolution is pH-dependent and oral absorption is greatest in the upper small intestine.

Feed premixes at 5% and 10% w/w potency require stepwise geometric dilution. Regional good manufacturing practice and feed regulations generally set mixer performance acceptance at a coefficient of variation below 5.0% across at least 10 sampling points. Ribbon mixers with working volume 60–70% capacity reduce segregation; vertical screw mixers are less suitable because the API accumulates in the lower cone when particle-size differences are large. Low-inclusion premixes below 1.0% usually require milled dihydrate with D90 ≤100 µm, while coarse material above 150 µm can produce sampling bias and uneven drug distribution in finished feed. Carriers based on limestone or calcium carbonate are avoided because oxytetracycline chelates calcium and may show reduced oral availability.

When Injectable Solutions Require Endotoxin Control Below 0.25 EU/mg

For parenteral aqueous solutions at 100 mg/mL or 200 mg/mL, oxytetracycline hydrochloride is dissolved under nitrogen with an antioxidant and pH adjuster. Sterilization is performed by filtration through 0.22 µm polyvinylidene fluoride or polyethersulfone membranes; terminal autoclaving is avoided because degradation to 4-epioxytetracycline and anhydrotetracycline is accelerated above 60°C. The bacterial endotoxin limit for a product dosed at 20 mg/kg is calculated from USP <85> as 0.25 EU/mg; depyrogenated glass vials, stoppers, and transfer lines are required. Contact surfaces should be 316L stainless steel or glass-lined because oxytetracycline chelates polyvalent metal ions, and iron or copper residues accelerate oxidative discoloration. For long-acting injectable suspensions containing 200 mg/mL oxytetracycline base, the dihydrate is micronized to control sedimentation volume and redispersibility; syringeability failure is the principal production defect observed when D90 exceeds 100 µm or when the vehicle viscosity is not adjusted to the selected needle gauge. Published data for specific vehicle composition and stability in high-temperature tropical storage are limited, but accelerated studies generally use 40°C and 75% relative humidity for 6 months according to ICH Q1A for non-aqueous or semi-solid systems.

Relative to doxycycline hyclate, oxytetracycline is more hydrophilic, has a shorter apparent elimination half-life in cattle and pigs, and depends on long-acting parenteral formulations or continuous feed or drinking-water administration to maintain effective plasma exposure. Doxycycline typically shows greater lipophilicity and tissue penetration, but its regulatory status in food-producing species is narrower in several jurisdictions. Oxytetracycline also chelates Ca²⁺, Mg²⁺, and Fe²⁺ more extensively in the gastrointestinal lumen; co-administration with milk replacer or mineral-rich diets can reduce oral bioavailability by more than 50% in pigs and calves. In contrast, beta-lactam antibiotics lack activity against Mycoplasma and Rickettsia; however, oxytetracycline is bacteriostatic and may antagonize the bactericidal activity of beta-lactams in infections when simultaneous cell-wall disruption and ribosomal inhibition are expected to be synergistic but are instead counterproductive. Compared with chlortetracycline, oxytetracycline has a different related-substance profile and different relative potency in feed premix assays; the two are not interchangeable without revalidation of the finished-feed method. Minimum inhibitory concentration susceptibility breakpoints vary by pathogen, tissue, species, and regulatory region; published data for direct clinical breakpoint comparisons in minor species are limited.

Does Oxytetracycline Retain an Advantage over Doxycycline in Feed Medication?

For group medication of swine, poultry, and calves, oxytetracycline is often selected under veterinary prescription when susceptibility testing supports use, because it is available in water-soluble and feed-premix grades and can be delivered at therapeutic levels of 10–20 mg/kg body weight per day by adjusting inclusion rate according to measured daily feed or water intake. Doxycycline has more limited approvals for feed medication and is generally more expensive; its oral absorption is less influenced by moderate dietary calcium in some species, but oxytetracycline remains commonly used in feed and water medication programs where broad Gram-positive and Gram-negative coverage plus Mycoplasma activity is required. Finished feed and medicated water should be prepared in accordance with regional veterinary prescription requirements, and withdrawal periods are defined by the finished-product marketing authorization rather than by the API manufacturer’s certificate of analysis. The API may be supplied with reduced heavy-metal content according to ICH Q3D and with residual solvent levels controlled to USP <467>; certificates of analysis should be reviewed for Class 1, Class 2, and Class 3 solvent status before releasing components for medicated feed use.

Stability of reconstituted oral solutions and medicated drinking water is limited by pH, light, and trace-metal contamination. Solutions should be protected from direct sunlight and preferably used within 24–48 h; prolonged storage at ambient temperature can increase turbidity and epimerization products. Water lines and tanks for medicated water should be non-metallic or 316L stainless steel to minimize chelation-induced precipitation.