Brimonidine L-tartrate, CAS 70359-46-5, is the L-tartaric acid salt of 5-bromo-6-(2-imidazolidinylideneamino)quinoxaline and is supplied as a white to off-white crystalline powder. The molecular formula is C11H10BrN5·C4H6O6 and the relative molecular mass is 442.2 g/mol. The product is assigned two control models: BT-O for tablet, capsule, and granule feedstocks, and BT-I for injectable solution, suspension, and low-endotoxin oral liquid intermediates. The two models share the same active moiety and salt stoichiometry; they differ in particle-size distribution, bioburden, endotoxin, residual solvent, and subvisible particulate controls.

The oral grade is released under Ph. Eur. 5.1.4 for non-sterile oral products; the injectable grade is released with endotoxin and subvisible particulate controls that are not required for oral grade. This route-linked control philosophy prevents use of an oral-grade powder in a parenteral compounding line where filter blinding and endotoxin breakthrough are known batch-failure modes.

For tablet and capsule development, the API is typically wet-granulated or dry-granulated at a drug loading between 0.5% and 10% w/w. At loading below 1.0%, direct compression is generally avoided unless the API is preblended with a carrier at a fixed 1:5 ratio in a tumble blender for 20 min; this reduces assay variability caused by electrostatic adhesion to the blender walls. For oral and injectable solutions, the API is dissolved at 2 mg/mL as active base equivalent; pH adjustment is usually not required if the tartrate salt is used, but the final pH must still conform to the finished product specification.

How Does the L-Tartrate Counterion Change Dissolution and Downstream Processability?

Salt formation with L-tartaric acid alters aqueous solubility and wetting relative to the free base. The salt dissolves readily in purified water at 2 mg/mL, whereas the free base requires pH adjustment for comparable dissolution. In oral dry blends, this change is observed as a reduction in wetting time; tablets containing the tartrate salt and a superdisintegrant such as croscarmellose sodium show a measurable drop in disintegration time when the API particle size is held at a D50 of 80–120 µm. The same solubility creates a segregation hazard in low-dose direct compression if the particle size is reduced below D50 40 µm without granulation. On a rotary tablet press with 25 kN precompression and 60 kN main compression stations, ejection force remains below 800 N when 0.75% w/w magnesium stearate is present and the compression suite is held below 40% RH. Compared with the hydrochloride salt, the L-tartrate form avoids chloride-induced pitting on stainless steel contact surfaces and exhibits lower hygroscopicity in open-container tests at 25°C/60% RH.

Particle engineering is not controlled by a single specification. For oral granule and direct compression, the coarse crystalline material is screened through a 0.5 mm sieve and blended to a laser-diffraction D50 of 80–120 µm with a D10 above 20 µm to limit dusting. For injectable-grade material, spiral jet milling under nitrogen at an injector gas temperature not exceeding 25°C produces a D90 of ≤25 µm and a D50 of ≤10 µm. Milled material is assayed for amorphous content by X-ray powder diffraction; the amorphous halo is kept below 5% w/w because amorphous domains accelerate hydrolysis of the imidazoline ring. The crystal form is confirmed by differential scanning calorimetry; the melting endotherm is recorded and must match the reference crystalline form within ±2°C onset. Particle-size analysis follows ISO 13320; powder flow is judged by Hausner ratio and angle of repose per Ph. Eur. 2.9.36.

Chiral Purity and Salt Stoichiometry Control in the L-Tartrate Backbone

Because L-tartaric acid is a chiral counterion, the absence of D-tartrate is release-relevant. The D-tartrate content is determined by chiral high-performance liquid chromatography and is controlled to ≤0.5% area normalized. Excess unbound L-tartaric acid is monitored by ion chromatography; the molar ratio of tartrate to brimonidine free base is 1.00:1.00 with a tolerance of ±0.03. Crystal lattice incorporation is confirmed by X-ray powder diffraction; a stoichiometric salt can be distinguished from a physical mixture by the presence of the full ionic salt pattern. Salt stoichiometry drift changes dissolution pH and increases hygroscopicity, so it is reported on every certificate of analysis.

When Injectable-Grade Brimonidine L-Tartrate Is Selected for Parenteral Manufacturing

Selection of the BT-I grade requires the CMC reviewer to distinguish between an API that is low in endotoxin and an API that is sterile. The BT-I grade is not sterile; it is a controlled bioburden powder intended for dissolution, filtration, and terminal sterilisation or aseptic processing downstream. Release testing includes bacterial endotoxin by kinetic turbidimetric LAL with a limit of <0.25 EU/mg according to Ph. Eur. 2.6.14 and USP <85>, total aerobic microbial count ≤100 CFU/g, and total combined yeasts and moulds ≤20 CFU/g according to Ph. Eur. 2.6.12/USP <61>. Subvisible particulate control is verified after reconstitution through a 0.45 µm filter using light obscuration per USP <788>; the user must confirm that the chosen final container closure meets the parenteral product monograph. Residual moisture is kept at ≤0.3% w/w because the molecule undergoes hydrolysis in the presence of free water.

Residual solvent and elemental impurity control follows the harmonised guidelines. The synthesis may retain class 3 solvents such as methanol, acetone, and ethyl acetate; their combined concentration is maintained below 0.5% w/w and reported using Ph. Eur. 2.4.24 or USP <467>. Elemental impurities are handled by a documented risk assessment under ICH Q3D; the control points include lead, cadmium, arsenic, mercury, and palladium when catalytic hydrogenation is used. Quantitation follows USP <233> and Ph. Eur. 2.4.20 as applicable. Nitrosamine risk is assessed as part of process and packaging studies; the imidazoline ring can act as a nitrosatable amine, so aqueous processing aids and packaging headspace are controlled.

Monitoring Granulation, Drying, and Compression to Limit Batch-to-Batch Drift

On a production-scale high-shear granulator with a 25 L bowl and 300 mm impeller blade, wet massing of a 5% w/w drug-loaded formulation is stopped when torque rises to 1.4–1.8 N·m. Transfer to a fluid-bed dryer is made at a product temperature of 28–32°C; drying is continued until loss on drying is between 1.0% and 1.5% w/w. Overdrying below 0.8% w/w increases granule friability and shifts the size distribution toward fines; this is observed as an increase in tablet capping at compression forces above 12 kN. The dried granule is milled through a 1.0 mm screen and blended for 15 min at 60% vessel fill. Tablet tensile strength is measured with a 5 kN hardness tester; a loss of 15–20% tensile strength after wet granulation is an early signal of solute migration of the highly water-soluble tartrate salt.

What Distinguishes This Product from Unspecified Brimonidine Base or Hydrochloride Sources?

The L-tartrate salt is not interchangeable with the free base or a hydrochloride salt without reformulation. In dissolution screening at 50 rpm in pH 6.8 phosphate buffer using USP apparatus II, the tartrate salt releases faster than the free base; the t85 shifts from more than 60 min to under 30 min when particle size is matched at D50 90 µm. The hydrochloride salt can release rapidly but may introduce chloride-associated volatility during wet granulation and pitting of stainless steel punches over multi-batch campaigns. The L-tartrate form is also less acidic in solution than the hydrochloride analogue, which allows closer pH matching to physiological buffers in injectable formulations. The product is controlled for D-tartrate, residual solvents, elemental impurities, and route-specific particle size; generic brimonidine base sources may lack the same route-of-administration controls.

Stability data for the API are generated in accordance with ICH Q1A(R2). Long-term storage is 25°C/60% RH for oral grade and 5°C for injectable grade; accelerated testing is 40°C/75% RH for 6 months. The product is packaged in double low-density polyethylene bags inside an aluminium foil laminate. Under these conditions, assay and related substances remain within specification for the assigned retest period; however the user must re-test after any breach of the moisture barrier. The tartrate salt should not be co-micronized with strong oxidizing agents or stored in direct sunlight because the active moiety is photolabile in solution.