Introduction
As a product, the TB vaccine candidate progresses from a design on paper and prototypes in a laboratory into a commercial product in a vial. The aim of this function is to define how the vaccine should be characterised at each stage, and to define the criteria for quality. There is an increased level of control on the specifications of the vaccine candidate as it progresses through the stages, to become a safe and effective product against TB. This includes specifications for the drug substance (the biologically active ingredients), the excipients, and the contaminants that are included in the final drug product, together with the assays to accurately monitor these quality attributes.
Vaccine technology specific considerations
The various vaccine platform products that serve as a basis for TB vaccine development have different product characteristics and quality attributes that need to be defined and controlled. These characteristics and attributes are often relevant for an entire vaccine technology platform and not necessarily specific for a TB vaccine. For example, the genetic stability of recombinant live-attenuated vaccines and viral vectors, reliability of inactivation of whole-cell killed vaccines, adjuvant attributes, and characterisation of nucleic acid-containing formulations (like LNPs for mRNA vaccines) are platform-specific considerations.
When specific adjuvants or excipients are added to subunit vaccines or DNA/ RNA based vaccines a rationale and data are needed to justify them. Also required are assays that show control on the quality of each component part of the new vaccine product. It must be shown that the combination of antigen, adjuvant and excipients form a stable, uniform Drug Product, which can be characterised with biological, chemical, and physical assay techniques. For DNA/ RNA based vaccines this characterisation must be done in combination with components in a stable formulation, such as liposomes for mRNA vaccines. For large, relatively undefined vaccines such as a modified strain of Mycobacterium tuberculosis as a live attenuated vaccine, the outcome of the pre-clinical and clinical testing is linked to the use of a specific cell bank. This is because no specific quality attributes can be monitored to demonstrate comparability of the vaccine product after a process change.
TB vaccine target population considerations
For each population or indication, developers should consider if extra focus is needed for specific product quality properties. For example, for the development of a TB vaccine for infants or immuno-compromised target populations (HIV+), the safety aspect will have more emphasis, likely resulting in additional restrictions for the impurity profile or attenuation. For the application in mass vaccination campaigns in tropical countries, there will be increased attention for the product stability in challenging cold chain conditions and the use of a simple and effective, preferably needle-free, system for vaccine administration.
- Screen and select antigen(s), adjuvants, other excipients and delivery system
- Characterise vaccine candidate. In particular, demonstrate antigen expression and purity (e.g. proteins)
- Antigens, adjuvants, other excipients, delivery system selected
- Characterisation tests defined and characteristics of the vaccine candidate documented
Discovery is the stage where the technology and design of TB vaccine candidates are made, and when TB antigens, adjuvants and delivery system are screened based on expression (yield) and immune response in animals.
Key for the characterisation of the drug substance are assays to measure its identity, potency, purity, and stability. These assays require specific development and depend on the type of vaccine, for example a TB subunit vaccine will require a different assay for identity than a recombinant BCG. Identification and expression of the substance often includes gel electrophoresis, western-blot, or Enzyme Linked Immunosorbent Assay (ELISA) for a protein, and Polymerase Chain Reaction (PCR) for recombinant DNA. Potency could be bacterial viability (colony-forming unit) for a recombinant BCG, coupled to a marker assay for an immunological relevant effect representing protection. For a subunit vaccine it could be the concentration linked to a protective immunological effect, and for a viral vectored vaccine it would be titre linked to a similar immunological output. For TB vaccines, T-cell epitopes and associated responses can be evaluated as markers for immunogenicity and potency. Examples include- antigen specific stimulation of Interferon gamma production, measured by ELISpot, other cytokines, specific T-cell populations such as polyfunctional T-cells. Purity assays measure contaminants that originate from the expression system and culture medium used, such as host cell protein, host cell DNA, endotoxin or serum components, or from the process conditions used, e.g. detergents (Triton) or DNAse (benzonase).
The characteristics ofmodified TB as a live attenuated TB vaccine have been published in the ‘GenevaConsensus’ publications (Geneva consensus and 2nd Geneva consensus).
The following guidelines on quality control of vaccine development provide further information: ICH Q8 Pharmaceutical Development, August 2009 (ICH Q8), ICH Q11 Development and Manufacture of Drug Substances (chemical entities and biotechnological/biological entities), May 2012 (ICH Q11) and ICH Q2, Validation of Analytical Procedures: Text and Methodology, Nov. 2005 (ICH Q2).
- Document vaccine product stability
- Select assays to monitor product quality, including an assay for potency
- Initial product stability defined
- Feasible assays for product quality attributes selected, including for potency
There are further characterisations of the vaccine substance / product for its identity, purity and potency. Evaluation of stability under the expected storage conditions must be performed at this stage, and this becomes a criterion for selection.
There is no correlate of protection for TB vaccines. This presents a challenge to determine the potency of a vaccine candidate. Beside its concentration, several different marker assays are used as indicated in Stage A. The recently developed mycobacterial growth inhibition assay (MGIA) could be considered as a surrogate measure of a protective immune response.
The assays required for the testing of Critical Quality Attributes (CQA), the most relevant criteria for product quality with respect to safety and efficacy, are selected.
- Optimise and finalise vaccine composition and route of administration
- Develop release assays and stability tests
- Prepare batch release
- Vaccine characteristics critical for vaccine immunogenicity (e.g., antigen sequence, adjuvant, delivery system, route of administration) finalised
- Target Bill of Testing (BOT) defined with target specifications for final product, drug substance (potency), excipients, impurity profile and product composition
- Quality control (QC) tests selected and their feasibility demonstrated, incl. for potency/ relevant biological activity, identity, purity, and stability
The assays selected for quality control and characterisation are developed, together with target specifications, for identity, potency, purity, etc. These quality parameters of the substance and the product are referred to a Bill of Testing (BoT), or Quality Target Product Profile (QTPP).
Note: the raw materials used for production of the substance and the product should be checked before being used in the lab. This quality control procedure is mandatory for current Good Manufacturing Practice (cGMP) operations. Certificates of Analysis of raw material must be collected and assays with target specifications selected for release.
- Release batch for the toxicology studies
- Qualify assays related to critical quality attributes (CQA, product) and critical process parameters (CPP)
- Release seed lots and banks
- Release drug substance (DS) and drug product (DP) for FIH/Phase 1
- Optimise buffer formulation and generate stability data on DS and DP
- Batch for toxicology studies released
- Critical assays qualified
- Seed lots and banks released
- Clinical DS and DP meet specifications and are released. Equivalence to batch used for toxicology confirmed
- Stability data on formulated vaccine sufficient to support clinical trial
In preparation for manufacturing of GMP material, meaning production of the antigen(s) and their formulation, the assays used for quality control and the Critical Process Parameters (CPP) must be qualified. The list of parameters that needs to be tested to release a product batch is confirmed and recorded in the Bill of Testing. Beyond this point the Product Specifications cannot be changed outside the original ranges without having major consequences (except for the excipients).
The material used for the toxicity study is produced according to cGMP or equivalent to GMP and must be characterised and released accordingly. The Phase I material is also produced and released.
- Fine tune (if needed) quality control (QC) assays
- Qualify and/or validate QC procedures
- Characterise newly produced drug substance (DS) and drug product (DP)
- Perform stability studies
- QC assays refined (if needed)
- Plan to ensure validation of QC procedures established
- DP and DS pass the tentative product specifications recorded in the Target Bill of Testing (BOT); impact of changes and deviations on DS and DP are documented
- Stability data sufficient to support clinical trial
During Phase 1, the assays for Critical Quality Attributes (CQA) continue their qualification, progressing to validation. The assays used for product characterisation could be slightly modified, to improve the sensitivity or accuracy of the test. This should not change the qualification or later validation status of the assays. Up to this point, the product reference would be from well described R&D production runs and be used to standardise assays. It could now be replaced by a GMP compliant reference.
The assays are used to release any new GMP material produced, or to document changes in the drug product (within certain limits), and in the continuing stability studies.
- Confirm specifications of final formulation, drug product (DP)
- Confirm criteria and specifications of DP manufactured for consistency, and define the Bill of Testing (BOT)
- Compare drug product (DP) used in different studies, as relevant
- Continue stability testing
- Define and validate final Quality Control (QC) analytical methods and assays, for the release of batches and final product
- Specifications for DP confirmed
- Criteria and specifications for consistency runs are finalised and the BOT is defined
- Comparability of products used in different clinical studies is available
- Stability acceptable
- QC assays are validated
- Release Good Manufacturing Practices (GMP) Phase 3 and consistency batches
- Document consistency between batches
- Evaluate stability data against TPP
- No major out-of-specs for the product as listed in the Bill of Testing (BOT)
- Consistency of batches documented
- Stability studies completed and data support TPP
Stability data have been generated from the first R&D lot and for each manufacturing lot. These stability data should be compared, as part of the characterisation of the product. A sufficient amount of vaccine product from the Phase 3 production has to be stored to serve as a long-term reference standard for QC product assays.