Recombinant Mycobacterium Tuberculosis Fusion Protein Pharma Grade API for Tablet / Capsule / Granule / Injection, Oral & Injectable is a recombinant biological active ingredient prepared from a genetically engineered Escherichia coli expression system carrying a defined fusion construct of immunodominant Mycobacterium tuberculosis secretory antigens. The construct commonly links early secretory antigenic target ESAT-6 and culture filtrate protein CFP-10 through a flexible glycine-serine spacer, with the mature protein purified by orthogonal chromatography and 0.2 µm filtration. The manufacturer differentiates lyophilized grade rMTB-FP-Lyo and liquid grade rMTB-FP-Liq; published pharmacopoeial monograph data for this specific fusion configuration is limited, so release methods are aligned with ICH Q6B and relevant Ph. Eur. general chapters. The API is supplied as a sterile injectable grade for parenteral compounding and as a bioburden-controlled oral solid-dose grade intended for enteric-coated tablet, capsule, or granule processing.

Pharmaceutical Grade Definition and Route-Specific Presentation

The lyophilized injectable grade is released as a white to off-white cake in Type I borosilicate vials with rubber stoppers and overseals. The liquid oral grade is a colourless to pale-yellow solution containing the fusion protein in a histidine or phosphate buffer with trehalose or mannitol as cryoprotectant. Specifications differ by intended route; a representative release profile is given in Table 1.

Because the API is a protein, the aggregate content is monitored by size-exclusion HPLC rather than only by electrophoretic purity. A product batch failing the injectable aggregate limit cannot be re-labelled as oral grade unless the oral grade also meets the higher endotoxin and bioburden limits; blending of rejected parenteral batches into oral campaigns is not permitted under ICH Q7.

Published process descriptions for similar E. coli-expressed recombinant subunit antigens indicate that dissolved oxygen is typically maintained above 30% air saturation and harvest is performed by continuous disk-stack centrifugation followed by tangential-flow microfiltration. In production-scale equipment, host-cell protein clearance rather than product titre is the principal batch-to-batch processing risk, because the fusion protein is expressed intracellularly and requires solubilisation and refolding before chromatographic purification.

What Separates Recombinant Fusion-Protein API from Native PPD and Single-Antigen rDNA Products?

The distinction is not only molecular composition but also control of identity and potency. Native PPD is a heterogeneous mixture of mycobacterial proteins derived from heat-killed culture filtrate; its composition varies between production batches and is standardised by skin-test reactivity rather than by molecular weight or host-cell protein content. A recombinant fusion-protein API contains one or more defined antigen sequences; identity is confirmed by liquid chromatography-tandem mass spectrometry peptide mapping, and potency can be expressed as antigen content per milligram rather than arbitrary tuberculin units.

For interferon-gamma release assay applications, the product differs from synthetic peptide pools because the fusion protein is a single covalent polypeptide rather than a mixture of overlapping oligopeptides. This structural distinction reduces the number of process-related impurities associated with solid-phase peptide synthesis and simplifies identity testing by intact-mass analysis. However, a fusion protein cannot be directly substituted into an assay validated for separate ESAT-6 and CFP-10 peptide pools without revalidation of the cut-off, sensitivity, and specificity.

Unlike live attenuated BCG, the recombinant fusion protein API is not replication-competent and does not require the same biosafety containment during sampling and blending. Nevertheless, it is not a directly substitutable immunogen for BCG without clinical equivalence data. The fusion design can also introduce linker-dependent neo-epitopes that are absent from native mycobacterial proteins; immunogenicity risk is therefore evaluated according to ICH S6(R1) for biological products.

When Oral Solid Dosage and Parenteral Injection Share the Same API

Oral and injectable presentations share the same purified fusion protein but diverge in formulation. For the oral route, the API is not directly compressible; the lyophilized cake has low bulk density and poor flow, and direct compression tableting under high compression force can induce particle bridging and capping. Roller-compacted granules containing the API with microcrystalline cellulose at 20–30% w/w, mannitol as diluent, croscarmellose sodium at 2–4% w/w, and magnesium stearate at 0.5–1.0% w/w are prepared at relative humidity below 40%. Tablet cores are then enteric-coated with methacrylic acid-ethyl acrylate copolymer to 7–10% weight gain, because unprotected fusion protein is degraded in simulated gastric fluid at pH 1.2 within 15 min of exposure.

Parenteral use requires reconstitution in water for injection or a compatible buffer; the solution is filtered through a 0.22 µm polyethersulfone filter before aseptic filling or clinical preparation. After reconstitution, the product is held at 2–8°C and used within 6 h to minimise subvisible particle formation. Subvisible particulate testing follows USP <787> with limits for ≥10 µm and ≥25 µm particles. Intramuscular or subcutaneous administration is the conventional route; intravenous use is not assumed and requires separate formulation and compatibility studies.

Fluid-bed granulation offers a processing window reported for heat-labile protein APIs: inlet air temperature 45–55°C, product temperature 28–32°C, and spray rate 8–12 g/min for a 5 kg batch using a top-spray insert. Under these conditions, monomer loss is held below 5% by SEC-HPLC. Enteric-coated granules are filled into hard gelatin capsules and tested by two-stage dissolution in 0.1 M HCl for 2 h followed by pH 6.8 phosphate buffer, with release ≥80% in 45 min per Ph. Eur. 2.9.3 or disintegration per Ph. Eur. 2.9.1. The oral and injectable presentations are not interchangeable pharmacokinetically, because oral administration of a protein delivers antigen to gut-associated lymphoid tissue rather than producing systemic exposure.

Residual Impurity Limits and Pharmacopoeial Method Assignments

Release testing for the injectable grade includes bacterial endotoxin by kinetic chromogenic method per Ph. Eur. 2.6.14, sterility per Ph. Eur. 2.6.1, and visible particles per Ph. Eur. 2.9.20. The oral solid-dose grade is tested for total aerobic microbial count and specified organisms per Ph. Eur. 5.1.8, but is not sterile. Host-cell protein contamination is measured by a validated broad-specificity ELISA and is controlled to ≤100 ng/mg; host-cell DNA is controlled to ≤10 ng/dose by quantitative polymerase chain reaction. Residual fermentation antibiotics such as kanamycin are controlled to the lowest validated detection limit; where an antibiotic-resistance marker is not used, the certificate of analysis states absence of the relevant β-lactam and aminoglycoside markers.

Process-related impurities from affinity-tag removal—if hexahistidine tags are employed—include residual imidazole and nickel leachate. The manufacturer’s testing panel may include imidazole by reversed-phase HPLC with a limit of ≤10 ppm and nickel by inductively coupled plasma mass spectrometry with a limit of ≤1 ppm. The liquid oral grade contains the same fusion protein but permits higher excipient-related residual monomers after terminal dilution; residual monomer control is performed by gas chromatography when methacrylic acid copolymer-coated granules are tested.

Because filtration does not remove endotoxin, depyrogenation of chromatography columns, buffers, and vialing components is mandatory. Ultrafiltration and diafiltration steps using 10 kDa or 30 kDa membranes reduce low-molecular-weight process impurities, but prolonged diafiltration can induce aggregation at the air-liquid interface; process records therefore include filter flux and transmembrane pressure limits.

Controlling Host-Cell Protein Clearance During Downstream Purification

During downstream purification, host-cell protein content is reduced from clarified lysate levels that can exceed 40,000 ppm to a final drug substance value of ≤100 ng/mg by sequential anion-exchange and hydrophobic interaction chromatography. Column loading is limited by dynamic binding capacity, and production batches may require two capture cycles when the fermentation titre exceeds 2 g/L; this is a known bottleneck in Escherichia coli-derived subunit antigen manufacturing. Protein A chromatography is not applicable because the fusion protein lacks an Fc domain; clearance of DNA is achieved by anion-exchange flow-through at pH 7.5–8.0 with conductivity below 10 mS/cm.

Sterile filtration is performed after final formulation to reduce bioburden and remove large aggregates, but the 0.2 µm filter is integrity-tested by bubble point at the manufacturer-specified pressure before use. Terminal storage of the lyophilized injectable grade is at 2–8°C in dark conditions; the liquid oral grade is stored at -20°C until granulation. In-use stability of the thawed liquid grade is limited to 72 h at 2–8°C. Repeated freeze-thaw cycles increase subvisible particles and aggregation; no more than 3 freeze-thaw cycles are recommended for the liquid oral grade before processing.