RESOMER RG 750 S Bioresorbable PLGA Drug Delivery Grade is a biodegradable poly(D,L-lactide-co-glycolide) copolymer with a nominal lactide/glycolide molar ratio of 75:25 and an inherent viscosity specification of 0.16–0.24 dL/g when measured as a 0.1% solution in chloroform at 25 °C in accordance with ISO 1628-1:2021. The material is supplied as a white to off-white granulate or powder intended for parenteral drug delivery systems, including solvent-evaporation microspheres, subcutaneous implants, and in-situ gel-forming depots. RG 750 S carries a carboxylic acid terminal group; this end-group chemistry accelerates water uptake and hydrolytic ester cleavage relative to ester-capped 75:25 PLGA grades of comparable molecular weight. The acid terminus can also influence loading of amine-containing active pharmaceutical ingredients through ion-pair formation during solvent exchange. Residual methylene chloride, when dichloromethane is used in downstream processing, must be controlled below the ICH Q3C class 2 limit of 600 ppm in the finished dosage form.

Specification Limits and Certified Test Methodology

The following table summarises the typical specification envelope for RG 750 S. These values should be read with the manufacturer’s certificate of analysis because release limits may vary by supply agreement.

Molecular weight by size-exclusion chromatography with refractive-index detection and polystyrene calibration generally places RG 750 S in the weight-average molecular weight range of 10,000–20,000 Da; however, this is a relative result and not an absolute molecular mass. The glass transition temperature is low enough to permit melt processing below 120 °C, but high enough to avoid room-temperature flow of solidified implants.

On a 10 L jacketed reactor equipped with a 4-blade pitched-turbine impeller, solvent-evaporation microsphere production using RG 750 S commonly employs an organic phase of dichloromethane at polymer concentrations between 5% and 20% w/v. The aqueous continuous phase contains poly(vinyl alcohol) at 1–2% w/v and is maintained at 15–25 °C. The emulsification step is sensitive to the low inherent viscosity of RG 750 S: when polymer concentration in the organic phase falls below 5%, the dispersed-phase viscosity may drop below 5 mPa·s, increasing the probability of satellite droplet formation and broad particle size distributions. Impeller tip speed is therefore kept in the range of 3.1–6.2 m/s; higher speeds produce submicron fragments carrying insufficient polymer mass to sustain release. After solvent evaporation, the hardened microspheres are washed with water for injection and lyophilised. Residual moisture is typically controlled below 0.5% by Karl Fischer titration because higher water content reduces glass-transition temperature and induces handling problems during terminal filling.

For hot-melt extrusion, RG 750 S can be processed on a 16 mm co-rotating twin-screw extruder with an L/D ratio of 40:1. Barrel temperatures between 60 °C and 90 °C are usually sufficient to maintain screw torque below 70% of drive capacity at screw speeds of 100–200 min⁻¹. The melt temperature must not exceed 120 °C for prolonged residence times because intramolecular transesterification can generate lactide and glycolide monomers that act as plasticisers and broaden molecular weight distribution. When the torque signal oscillates by more than ±15%, feed bridging or local melt viscosity drop should be investigated. Nitrogen blanketing of the feed hopper is recommended when ambient relative humidity exceeds 60% RH, as moisture uptake during extrusion accelerates hydrolytic chain scission and reduces implant mechanical integrity.

Why Does Carboxylic Acid Termination Accelerate Hydrolysis in 75:25 PLGA?

In aqueous media, the free carboxylic acid terminus in RG 750 S lowers the local pH within the polymer matrix and catalyses ester-bond hydrolysis. Bulk erosion proceeds more rapidly than in an ester-capped 75:25 PLGA of equivalent inherent viscosity because the ester-capped grade lacks the additional acid source and hydrates more slowly. At pH 7.4 and 37 °C in phosphate-buffered saline, the acid-terminal effect is most pronounced in the early phase of degradation before autocatalytic internal acid accumulation equalises the pH gradient. This shifts the in vitro release curve of drugs with pH-dependent solubility; basic drugs that are protonated and entrapped as salts within the acidic PLGA matrix may show lower initial burst release than neutral drugs.

The 75:25 lactide/glycolide ratio contributes to a longer degradation interval than 50:50 PLGA grades. The additional methyl side chains on lactide units impede water penetration and reduce hydrolysis rate constants, so RG 750 S is applied where month-scale release is required rather than the faster erosion window typical of 50:50 grades. The following comparative table positions RG 750 S against common related grades. Relative erosion rates are based on the same particle size, molecular weight band, and buffer environment; direct comparison across different particle sizes is not appropriate.

When the intended route is an in-situ forming depot, RG 750 S is dissolved in N-methyl-2-pyrrolidone or dimethyl sulfoxide at 20–40% w/w and injected into an aqueous physiological environment. The solvent exchange precipitates the polymer, forming an implant with a highly porous structure. The low inherent viscosity of RG 750 S reduces the force required for injection through a 21 G needle compared with higher-molecular-weight 75:25 PLGA grades. However, the reduced chain length also produces a less viscous depot after precipitation, and the initial burst release is frequently higher than that observed with a 0.45–0.60 dL/g grade. Formulators compensate by increasing polymer concentration, adding a hydrophobic plasticiser, or using a rate-limiting membrane coating.

Microsphere encapsulation of water-soluble peptides and small molecules with RG 750 S commonly uses water-in-oil-in-water double emulsion. The primary emulsion is generated with a rotor-stator homogeniser at 8,000–20,000 min⁻¹; the secondary emulsification uses lower shear. Because RG 750 S solidifies quickly during solvent extraction, encapsulation efficiency for highly water-soluble compounds is more strongly influenced by primary emulsion viscosity than by continuous-phase pH. Poly(vinyl alcohol) concentration above 2% w/v reduces droplet coalescence but increases residual surfactant on the particle surface and may require additional washing steps.

If Primary Amine-Containing Excipients Are Blended with RG 750 S

Primary amines and strongly nucleophilic additives are incompatible with RG 750 S in melt and solution processing because they cleave the polyester backbone through aminolysis. This reaction is undesirable during hot-melt extrusion and can be identified by a rapid increase in melt flow rate or a drop in screw torque. In solution, the same aminolysis pathway reduces molecular weight during long hold times and can alter in vitro release kinetics before encapsulation is complete. Nucleophilic buffers such as tris(hydroxymethyl)aminomethane should not be used as the primary aqueous phase during solvent evaporation with this polymer; phosphate or acetate buffers at pH 7.0–7.4 are preferred. Storage under nitrogen at −20 °C is recommended for long-term stability. Once the container is opened, the polymer should be re-sealed with desiccant and held at 2–8 °C if consumed within 30 days. If exposure to 60% RH occurs for more than 24 h, vacuum drying at 40 °C for 12–24 h is required before hot-melt extrusion; lyophilised formulations should be kept below 0.5% moisture by Karl Fischer measurement.

Residual solvent control is both a regulatory and a processing limitation. When dichloromethane is used for microsphere manufacture, the residual level must comply with the ICH Q3C class 2 limit of 600 ppm in the final drug product, and typical in-process controls aim below 300 ppm before release. Ethyl acetate, if substituted, falls under class 3 and carries a higher permitted concentration of 5,000 ppm, but its lower volatility and higher aqueous solubility require extended vacuum stripping. For sensitive peptide-loaded particles, terminal gamma irradiation above 25 kGy is not routinely recommended because chain scission increases the proportion of low-molecular-weight polymer and accelerates release; published data for this specific configuration is limited and requires product-specific validation.

In dissolution testing with USP Apparatus 4 flow-through cells at 37 °C and pH 7.4, RG 750 S microspheres typically exhibit a biphasic release profile: an initial burst during the first 24 h, a diffusion-controlled lag phase, and a later erosion-accelerated phase. The burst phase is sensitive to particle size and surface porosity; spray-dried particles with specific surface area above 10 m²/g by nitrogen adsorption may release 20–30% of the payload within 6 h, whereas dense solvent-evaporation microspheres with residual solvent below 300 ppm can maintain the burst below 10%. The release-rate inflection point coincides with mass loss of roughly 5–10% and an internal pH drop below 3.0. This behavior distinguishes RG 750 S from higher-lactide or higher-molecular-weight PLGA products that exhibit longer lag periods and lower initial release under identical process conditions.