Serum gonadotrophin (PMSG, equine chorionic gonadotropin, eCG) pharma grade API is released as a lyophilized or freeze-dried glycoprotein fraction with an apparent molecular mass of 64–70 kDa and a high sialic acid content that extends circulatory persistence in cattle to approximately 40–50 h. The active substance is obtained from pregnant mare serum and purified through precipitation and chromatographic steps; dried API normally requires storage at 2–8 °C unless the manufacturer has validated an alternate thermal profile. PMSG is not a single molecular species but a distribution of isoforms with variable terminal sialylation, which affects both receptor interaction and in vivo half-life. Downstream sectors include injectable estrus synchronization and superovulation in cattle, sheep, and goats; lyophilized multi-dose injection kits for fixed-time artificial insemination; reference standards for potency bioassays; and limited dry-blend feasibility work for oral tolerance studies. The injectable route is the established pharmacologically effective route, while tablet, capsule, and granule formats encounter proteolytic and absorption barriers that confine them to experimental matrices, reference standards, or method-validation studies rather than systemic veterinary products.

Estrus Synchronization and Superovulation in Bovine Donor Herds: Subcutaneous Injection Formulation with Prostaglandin F2α Co-Administration

Standard superovulation protocols in Bos taurus and Bos indicus donor herds require subcutaneous or deep intramuscular injection of PMSG at doses between 1500 IU and 3000 IU per animal, depending on parity, body condition score, and previous stimulation history. The injection is prepared as a sterile aqueous solution with a pH of 6.8–7.2 and osmolality adjusted to 285–315 mOsm/kg using sodium chloride or mannitol. A progestogen-releasing intravaginal device is inserted on day 0; PMSG is administered at device removal or concurrently with prostaglandin F2α, most often 48 h before device withdrawal, to recruit a cohort of antral follicles. On production-scale filling lines, the filtrate is passed through a 0.22 µm PVDF membrane filter under nitrogen pressure into Type I borosilicate vials; hold time between sterile filtration and lyophilization or capping must not exceed 8 h at 2–8 °C to prevent aggregated species. The dosing window is narrow: administration more than 72 h before the expected ovulatory LH surge can generate unovulated follicles and luteal insufficiency, while administration after estrus onset produces inconsistent superovulation. Repeated treatment cycles exceeding three in the same animal are associated with anti-eCG antibody formation and reduced ovarian response; this is an operational boundary in elite embryo donor programs.

Field data from embryo transfer centers indicate that PMSG-based protocols reduce labor compared with twice-daily pituitary FSH injections, but the yield of transferable embryos in high-genetic-merit donors may be lower than that achieved with a descending-dose FSH schedule. GMP-licensed veterinary product manufacturers therefore position PMSG injection kits for single-injection superovulation in moderate-response herds rather than as a replacement for FSH in elite donors. The injection fluid must be free from mechanical particles because Type I glass vials with stored PMSG can shed silicone oil microdroplets from elastomeric closures if the rubber formulation lacks a fluoropolymer coating; silicone oil above 0.1 mg/mL can increase protein aggregation after reconstitution.

What Limits the Oral Route for PMSG Tablets, Capsules, and Granules Despite the API Specification?

Oral tablet, capsule, and granule delivery of PMSG faces first-pass proteolysis and low membrane permeability. The molecule has multiple disulfide bonds and N- and O-linked glycans; in the acid environment of the monogastric stomach at pH 1.2–2.5, subunit dissociation and sialic acid cleavage are rapid, while in ruminants microbial peptidases in the reticulorumen degrade the protein before the small intestine. A tablet or capsule cannot provide systemic gonadotropic activity unless a validated enteric coating, a peptidase inhibitor, and an absorption enhancer are combined; published data for this specific configuration with PMSG is limited. For granule dosage forms intended for research diets, the API is dispersed by geometric dilution with lactose monohydrate or mannitol, but blend uniformity is difficult because PMSG particles are electrostatic and exhibit poor flow. Direct compression of a PMSG reference tablet requires a moisture content below 1.5% w/w in the granulate; at higher residual moisture the glycoprotein undergoes aggregation and potency loss during accelerated stability storage at 40 °C and 75% relative humidity. No systemic oral PMSG product is described in current pharmacopoeial monographs as an approved alternative to injection.

Processing constraints in dry granulation are not negligible. Roller compaction pressure above 4 kN/cm can shear the glycoprotein and create fines, which drift during tablet compression and cause low content uniformity. Capsule filling with PMSG and microcrystalline cellulose requires a tamping-pin station speed below 60% of maximum to avoid static charge buildup. These constraints confine oral forms to exploratory studies; they are not a production-scale downstream market for systemic veterinary indications.

Lyophilized Cake Engineering for Multi-Dose Injection Vials

Lyophilized PMSG vials require a formulation that prevents ice-induced subunit dissociation and maintains cake integrity. A typical aqueous fill contains trehalose dihydrate 4.0–6.0% w/v as cryoprotectant, mannitol 2.0–4.0% w/v as crystalline bulking agent, and L-histidine buffer 10 mM at pH 7.0; polysorbate 20 at 0.01–0.02% w/v is added to reduce surface damage during filtration and filling. The glass transition temperature of the maximally freeze-concentrated solution depends on the buffer and cryoprotectant ratio; collapse temperature is typically between −18 °C and −22 °C for sucrose/trehalose-based formulations, so primary drying product temperature must remain below −25 °C. A production cycle can be run with shelf cooling at 0.5 °C/min to −45 °C, annealing at −12 °C for 3 h, primary drying at −25 °C shelf and 0.15 mbar for 36 h, and secondary drying at 35 °C and 0.05 mbar for 8 h. Residual moisture must be below 2.5% w/w by Karl Fischer method; a collapsed cake or a residual moisture above 3.0% w/w is a batch rejection trigger because it accelerates deamidation and aggregate formation.

Multi-dose injection kits are reconstituted with water for injection to a final PMSG concentration of 200–400 IU/mL. The reconstituted solution is inspected for particulate matter according to Ph. Eur. method 2.9.19; opalescent or fibrous material indicates protein aggregation. The lyophilized cake should have a uniform white or off-white appearance, no meltback, and a top surface that resists vibration-induced fracture during distribution. Batch-to-batch variability in drying rate is observed when the chamber load exceeds the condenser capacity, leading to increased product temperature and subtle cake collapse at the vial edge; this is a recognized scale-up failure mode.

Liquid PMSG injection batches that are not lyophilized are sensitive to adsorption losses on filler surfaces. During closed processing, PMSG at 50–100 IU/mL adsorbs to stainless steel and silicone pump tubing; recovery from the first 200 mL of filtrate can be 5–15% lower than the bulk solution unless the line is pre-equilibrated with a dilute albumin or surfactant solution. For this reason, liquid PMSG products are often filled in a carrier containing 0.1% w/v bovine serum albumin or 0.02% w/v polysorbate 20, but albumin addition creates a risk of adventitious-agent contamination and is rarely used in modern GMP lines. The fill volume is typically 1–2 mL per vial or pre-filled syringe; overfill of 0.05–0.10 mL is required to compensate for needle hub retention.

A Narrow Dosing Window Governs Small Ruminant Timed Artificial Insemination

In sheep and goats, PMSG is used for estrus induction during anestrus or for fixed-time artificial insemination after synchronization with progestogen-impregnated intravaginal sponges. The dose is generally 300–750 IU per animal administered intramuscularly at sponge withdrawal; in dairy goats, 400–500 IU is the most common field dose, while in ewes 300–600 IU is used depending on breed and season. Administration routes must avoid the sciatic nerve; a 20-gauge, 25 mm needle is used for an injection volume of 1–2 mL. Timed artificial insemination is performed 48–56 h after sponge removal; PMSG has a prolonged effect, so animals not detected in estrus may still be inseminated at a fixed time with acceptable pregnancy rates. The narrow dosing window is critical: doses above 750 IU in small ruminants increase the risk of multiple ovulations and early embryonic loss, while doses below 250 IU in lactating anestrous ewes often fail to induce luteal activity. A further operational boundary is the production of anti-eCG antibodies after repeated use across seasons; antibody formation reduces the response in subsequent treatments, so many flock programs restrict PMSG administration to one or two cycles per animal per breeding season.

Formulation consistency between batches is essential in small ruminant protocols because the therapeutic index is relatively narrow. A freeze-dried vial reconstituted with 2 mL water for injection yields a solution of 200–400 IU/mL; syringe dosing inaccuracies of 0.1 mL therefore represent 20–40 IU, which is biologically meaningful in small ewes. Producers are advised not to dilute beyond the manufacturer’s stated volume, because low-concentration solutions lose potency on standing due to adsorption to glass and plastic syringes.

When PMSG Potency Cannot Be Determined by HPLC Alone: Bioassay Standardization in Quality Control

Potency assignment for PMSG API and finished injections cannot rely solely on physicochemical methods because glycosylation heterogeneity affects in vivo activity. The pharmacopoeial standard, Ph. Eur. monograph 0713 for serum gonadotrophin for veterinary use, specifies an in vivo bioassay comparing ovarian weight gain in immature female rats after repeated subcutaneous injections against the International Standard for serum gonadotrophin. The test requires a defined animal housing environment: lighting cycle 14 h light and 10 h dark, temperature 21 ± 2 °C, and feed free of soy-derived phytoestrogens that could alter baseline ovarian weight. The routine acceptance range for potency is 80%–120% of the labelled claim, with a confidence interval typically 64%–156% depending on the number of animals per dose group. Bioassay failure is mainly caused by animal stress, poor injection technique, or reconstitution in a vehicle that denatures the glycoprotein.

For tablet, capsule, and granule experimental batches, sample extraction before bioassay uses phosphate-buffered saline pH 7.4 containing 0.1% w/v bovine serum albumin and 0.05% w/v polysorbate 20 to limit surface adsorption. The extraction is performed at 2–8 °C under gentle agitation; vigorous vortexing or sonication above 10 min can shear the glycoprotein and produce falsely low potency. The extracted API is filtered through a 0.45 µm low-protein-binding PVDF membrane prior to dosing. This bioassay-centered release profile applies equally to lyophilized injection vials and to dry blends used for reference standard qualification.

Dry granulation of PMSG with microcrystalline cellulose and crospovidone is not a registered production route for systemic veterinary products, but it is used to prepare reference tablets for method validation and stress studies. The granulate is produced by roller compaction with a gap of 1.5 mm and a roll pressure of 3 kN/cm; the resulting ribbons are milled through a 0.8 mm screen. Tablet compression is performed on a rotary press with a turret speed of 30 rpm and a pre-compression force of 2 kN, avoiding excessive main compression force above 6 kN that can induce compaction-induced aggregation. Capsule filling is limited to semiautomatic equipment with low-shear auger force; high-speed tamping pins generate electrostatic charges and reduce content uniformity below 85%. These processes are sensitive to ambient relative humidity; above 60% RH the powder becomes tacky and requires pre-drying of excipients at 40 °C for 2 h. Published data for the absolute oral bioavailability of PMSG from such matrices is limited, so these forms remain confined to laboratory-scale investigation rather than clinical veterinary use.