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Raw Materials in Gene Therapy — The Upstream Control Gap That Follows You to BLA

Starting MaterialsSpecificationsImpurity ControlBiologicsGene Therapy

In gene therapy manufacturing, impurity profiles are largely determined before fermentation begins. The raw material and starting material strategy you define at IND becomes the compliance liability you manage at…

By Khaled Aamer, PhD · Founder, XGene LLC Aug 22, 2026 13 min read

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Raw Materials and Starting Materials in Gene Therapy CMC: The Upstream Control Gap That Follows Your Product to BLA

That sentence is not a cautionary generalization. It is a mechanistic statement about how gene therapy manufacturing works. The impurities that end up in your final drug substance — host cell DNA, host cell proteins, process-related lipids, adventitious agents, residual solvents — trace their origins upstream, often to materials introduced before your manufacturing process technically begins. The quality ceiling of your drug product is set not at fill-finish, not at downstream purification, but at the moment you decide which plasmid DNA, which cell culture media components, which transfection reagents, and which ancillary materials will enter your manufacturing platform. Everything downstream manages inherited risk. Nothing downstream eliminates it entirely.

CBER has been explicit on this point. The FDA CMC Information for Human Gene Therapy INDs Guidance (2020) identifies raw material characterization and control as a foundational element of the CMC package expected at IND, and it ties the adequacy of that characterization directly to the acceptability of the drug substance specification. If your plasmid DNA lot used for GMP manufacturing is inadequately characterized, the downstream analytical package cannot compensate — because you do not have a qualified reference against which to measure degradation, contaminants, or batch-to-batch drift. The question CBER asks is not merely whether your final drug substance specification includes residual host cell DNA. The question is whether the upstream starting material — the plasmid — was itself manufactured under conditions that give you confidence in the identity, purity, and genetic integrity of what entered your process.

Plasmid DNA as a Critical Raw Material — The GMP Standard CBER Expects

Plasmid DNA occupies a structurally unusual position in gene therapy manufacturing. Under 21 CFR 211.101 and CBER’s interpretive framework, plasmid DNA used for the production of a gene therapy vector — whether that vector is an AAV, lentivirus, adenovirus, or non-viral construct — is classified as a critical raw material. It is not the drug substance. It is not a starting material in the ICH Q11 sense applicable to small molecules. But it is the genetic template from which your entire product derives its identity, and CBER treats it accordingly.

The practical consequence of this classification is that plasmid DNA must be manufactured under GMP, and the manufacturing site must be capable of providing a Certificate of Analysis (CoA) that supports not just identity and purity but the full profile of attributes known to affect downstream vector manufacturing performance. What does that profile look like? The technical standard that CBER expects, and that experienced CMC practitioners use as the baseline for supplier qualification, includes the following: supercoiled monomer content of at least 80%, assessed by capillary gel electrophoresis (CGE) or gel densitometry — this is not a cosmetic specification, because open circular and linear forms integrate into host cell genomes at different efficiencies and can generate distinct immunogenic profiles; residual host cell genomic DNA below 10 ng per milligram of plasmid DNA, measured by quantitative PCR, because host cell DNA carried into transfection introduces competing templates and contributes to the total residual DNA burden in the final drug substance; endotoxin below 5 EU per milligram of plasmid, determined by Limulus Amebocyte Lysate (LAL) assay, because endotoxin at higher levels suppresses transfection efficiency and independently constitutes a safety risk; residual RNA absent by gel electrophoresis, since RNA co-purification signals inadequate RNase treatment or downstream processing failure; identity confirmed by restriction enzyme mapping and full sequence confirmation, because point mutations, deletions, or recombination events in the plasmid directly compromise vector potency and introduce unpredictable transgene expression; and bioburden not exceeding 10 CFU per 10 mg of plasmid, demonstrating adequate environmental control at the manufacturing site.

These specifications are not aspirational. They are the working baseline against which CBER reviewers evaluate the adequacy of the plasmid manufacturing section in your IND CMC package. If your CoA does not include supercoiling percentage, does not report residual genomic DNA by a quantitative method, or lists only identity by agarose gel without sequence confirmation, you are not describing the quality of the material that entered your manufacturing process. You are describing a partial characterization that leaves unresolved the question of whether the genetic template is what you believe it to be, in the structural form required for consistent performance.

The supplier qualification obligation follows directly from this classification. ICH Q7 — the GMP guidance for active pharmaceutical ingredients, applied by CBER to critical raw materials in biologics manufacturing — requires that suppliers of critical materials be qualified through documented assessment of their quality system, manufacturing controls, and change notification procedures. For plasmid DNA, this means that the qualification file must include at minimum an audit record showing the manufacturing site was assessed against a defined GMP standard, a change notification agreement that obligates the supplier to inform you of any changes to the plasmid construct, the bacterial strain, the fermentation process, or the purification process, and a review of the supplier’s validation data for the specific plasmid lot or production campaign supporting your clinical material. This file must exist before the plasmid lot is used in GMP manufacturing, not assembled retrospectively when CBER asks for it.

The IND-stage temptation is to accept research-grade or “GMP-like” plasmid from an academic core facility or early-stage CDMO without completing the qualification process, on the grounds that the clinical program is not yet in late-stage development. CBER’s CMC guidance for gene therapy INDs does not provide that latitude. The guidance requires that the quality of the starting materials and raw materials used to manufacture clinical material be described and controlled from IND filing, and it explicitly notes that inadequate control of starting materials is a common deficiency observed in IND submissions. The upstream control gap opened at IND is not a temporary gap that closes when you move to Phase 3 manufacturing. It is a documentation debt that accumulates with every GMP lot manufactured from inadequately qualified plasmid, because you cannot retroactively requalify the material that built your clinical safety and efficacy database.

Cell Culture Media Components and Biologics-Derived Materials — The TSE/BSE and Viral Safety Obligations

The cell culture media used to grow your producer cell line — whether that line is HEK293, Sf9, BHK-21, or a proprietary derivative — introduces a second tier of raw material risk that is categorically distinct from the plasmid risk. Plasmid DNA risk is fundamentally about genetic identity, structural form, and microbial contamination. Cell culture media risk is about viral safety and transmissible spongiform encephalopathy (TSE) / bovine spongiform encephalopathy (BSE) exposure — risks that cannot be fully mitigated by downstream processing and that require source-level control from the beginning of the manufacturing program.

The regulatory framework here is layered and demanding. ICH Q5A(R2) — the guideline on viral safety evaluation of biotechnology products derived from cell lines of human or animal origin — requires that all materials of animal origin used in the manufacture of biological products be assessed for viral contamination risk. The FDA Guidance on Use of Materials of Animal Origin in Manufacture of Biological Products reinforces this requirement and specifies that the risk assessment must address the geographic origin of the animal-derived material, the species of origin, the tissue source, the manufacturing process used by the supplier, and the viral reduction steps applied. The EMA/CAT Guideline on quality of ATMPs adds specific expectations for gene therapy and cell therapy products, recognizing that the patient population — often pediatric, often severely immunocompromised — has heightened vulnerability to adventitious agent exposure.

The practical focus for most gene therapy programs is fetal bovine serum and bovine serum albumin. FBS, if used in the production cell culture step, requires a TSE/BSE risk assessment that documents the country of origin of the donor animals, confirms compliance with applicable regulations restricting the use of materials from BSE-affected herds, and identifies the serum fractionation process. Bovine albumin used as a stabilizer or cell culture supplement carries the same risk profile. CBER’s position, consistent with the FDA guidance on animal-derived materials, is that plasma-derived bovine albumin in gene therapy manufacturing is an addressable risk only if the risk assessment is complete, the supplier documentation is current and audited, and the program has evaluated whether a recombinant alternative is feasible. Recombinant human serum albumin — such as Recombumin, produced in Saccharomyces cerevisiae — eliminates the TSE/BSE risk entirely and removes the requirement for viral clearance studies attributable to this component. For gene therapy programs where the intended patient population includes immunodeficient pediatric patients, the shift from plasma-derived to recombinant albumin is not merely a quality preference. It is a scientifically defensible and increasingly expected risk management decision.

Beyond serum and albumin, the media compendial components — L-glutamine, sodium bicarbonate, DMEM base formulations — require testing against their compendial monographs. USP/NF standards for these components are well-established, and lot release testing against USP/NF specifications satisfies the baseline requirement. The supplier CoA must reference the applicable monograph, and incoming QC testing must confirm identity and purity before the material is released for use in GMP manufacturing. These are not high-risk materials, but they are materials where a documentation lapse — accepting a non-compendial lot, failing to document incoming testing, or using an expired lot — generates a GMP observation that is disproportionate to the underlying material risk.

Ancillary Manufacturing Materials — From Benzonase to Poloxamer — the Qualification Gap Most Programs Miss

The third tier of raw material risk in gene therapy manufacturing is occupied by ancillary materials — reagents and excipients that are not biological in origin, are not derived from animal tissues, and are not genetic templates, but that interact with the drug substance at process-critical steps and whose quality attributes directly affect the purity, safety, and stability of the final product. This category includes Benzonase endonuclease, polyethylenimine (PEI-MAX), dimethyl sulfoxide, poloxamer 188, and human serum albumin used in the formulation buffer. Each carries a distinct qualification obligation, and the gap between what most early-stage programs have on file for these materials and what CBER expects to see at BLA is substantial.

Benzonase endonuclease, manufactured by Merck/MilliporeSigma, is used in gene therapy manufacturing to digest host cell DNA and RNA during downstream processing. Its use is ubiquitous and its function is well-validated. But Benzonase is a biological reagent — it is a recombinant protein expressed in E. coli — and when it is used in drug substance synthesis, it is subject to the same viral clearance validation obligation that applies to any process-related biological material that contacts the drug substance and is not fully removed by subsequent processing steps. CBER’s expectation, consistent with ICH Q5A(R2), is that if Benzonase is not fully cleared from the drug substance by the downstream purification process, a viral clearance study must be conducted to demonstrate that the manufacturing process can achieve adequate reduction of viral contaminants potentially introduced by this reagent. The Benzonase lot release CoA must be GMP-grade, issued by the manufacturer with quantitative activity, purity, and endotoxin data, and the lot must be qualified before use in GMP manufacturing. Accepting a research-grade Benzonase lot for GMP manufacturing because the manufacturing team is confident in the downstream clearance steps is a qualification gap that CBER will identify.

PEI-MAX, the branched polyethylenimine formulation used for transient transfection in HEK293-based manufacturing processes, requires a transfection-grade purity specification and documented lot-to-lot consistency data. The criticality of PEI-MAX as a raw material is directly tied to its effect on transfection efficiency — the yield and quality of your vector production run are a function of the PEI-to-plasmid ratio, the molecular weight distribution of the PEI polymer, and the buffering capacity of the transfection complex. Lot-to-lot variability in PEI-MAX that is not detected by incoming QC testing will manifest as variability in vector yield and, potentially, in the ratio of full to empty capsids in AAV manufacturing. The specification for PEI-MAX must include not only chemical identity and endotoxin but a transfection efficiency confirmation — a cell-based assay using a standardized plasmid and reporter system — that confirms the functional performance of each lot before it is released for use in GMP manufacturing.

Dimethyl sulfoxide, used as a cryoprotectant in cell banking and occasionally in final drug product formulations, must meet pharmaceutical-grade USP standards. Where DMSO appears in the final drug product, residual solvent testing per ICH Q3C must be performed and the residual level reported in the drug product specification. Poloxamer 188, used as a surfactant in formulation buffers for gene therapy products, must meet NF grade standards, and particulate testing per USP <788> is required to confirm that the excipient does not contribute unacceptable particulate load to the final drug product. Human serum albumin, when used in the formulation buffer as a stabilizer — as it commonly is in AAV and lentiviral vector drug products — carries the same plasma-derived versus recombinant distinction described for cell culture albumin. Recombinant HSA eliminates the plasma-derived viral safety obligation. Where plasma-derived HSA is used, a viral reduction clearance study is required as part of the CMC package.

The unifying principle across all three tiers is forward propagation. Every qualification gap left open for a raw material at IND propagates forward through every GMP manufacturing lot, every clinical batch, and every comparability study conducted in support of BLA. The raw material and starting material control strategy is not a background section of the CMC package. It is the document architecture on which the credibility of your entire clinical manufacturing program rests.

THE XGENE GT RAW MATERIAL RISK AND CONTROL REGISTER

The XGene GT Raw Material Risk and Control Register is a tiered, risk-based tool used to classify all gene therapy manufacturing raw materials by criticality, GMP requirement, and regulatory compliance gap. It is applied during IND CMC package development to build the raw material section and prioritize supplier qualification activities before GMP manufacturing begins.

The Register operates across five tiers:

1. TIER 1 — GENETIC TEMPLATES (Plasmid DNA, Viral Seed Stocks): Highest criticality. GMP manufacturing required at the supplier site. Full characterization CoA required including supercoiling percentage, residual host cell genomic DNA by qPCR, endotoxin, residual RNA, identity by restriction mapping and full sequencing, and bioburden. Supplier qualification file must include audit record, change notification agreement, and lot-specific validation data. Compliance gap assessment: Is the manufacturing site qualified? Is the CoA complete against all required attributes? Is the qualification file current?

2. TIER 2 — ANIMAL-DERIVED BIOLOGICAL COMPONENTS (FBS, BSA, Plasma-Derived HSA): High criticality. TSE/BSE risk assessment required per ICH Q5A(R2) and FDA animal-origin materials guidance. Supplier documentation must include geographic origin, tissue source, and fractionation process. Recombinant alternatives must be evaluated and their use or non-use justified in writing. Compliance gap assessment: Is the risk assessment complete and current? Has a recombinant alternative been evaluated? Are plasma-derived lots subject to viral clearance attribution?

3. TIER 3 — PROCESS-CONTACT BIOLOGICAL REAGENTS (Benzonase, Recombinant Proteins): High criticality. GMP-grade CoA required. If residual reagent is not fully cleared from drug substance, viral clearance validation required. Compliance gap assessment: Is the lot GMP-grade? Has clearance been demonstrated or is a clearance study in the queue?

4. TIER 4 — PROCESS REAGENTS WITH FUNCTIONAL CRITICALITY (PEI-MAX, Lipid Transfection Reagents): Medium-high criticality. Chemical specification plus functional lot release assay required. Lot-to-lot consistency data must be on file. Compliance gap assessment: Is there a functional release assay? Is lot-to-lot variability characterized?

5. TIER 5 — COMPENDIAL AND FORMULATION EXCIPIENTS (L-Glutamine, NaHCO3, DMSO, Poloxamer 188): Medium criticality. Compendial testing per USP/NF required. Pharmaceutical-grade designation required for excipients. Where applicable: residual solvent testing per ICH Q3C; particulate testing per USP <788>. Compliance gap assessment: Are lots being tested against compendial monographs before use? Are CoAs current and supplier-specific?

The Register outputs a prioritized qualification activity list, a supplier audit schedule, and a gap narrative structured for insertion into the raw material section of the IND CMC package.