The criteria to select the production technology, beside in-house knowledge and experience, should be the target quality and quantity of the vaccine to be produced. It will influence the decision for the biological expression system and associated target production process that can deliver the target product criteria. Different production processes can be evaluated for suitability with a non-standardised monitoring of product quantity and quality (research quality). The most critical step is the selection of the biological expression system, as it determines the upstream part of the process and quality features of the product.

An initial step is the selection of strain(s). It is important to document the history of the strain as it will become the source of the Master Cell Bank (MCB) and Working Cell Bank (WCB), or the Master Virus Seed (MVS) and Working Virus Seed (WVS) for viral vectors.

A target cGMP compliant process exists for BCG suspension cells and this process could be used as the platform technology for the development of a production process for modified BCG and attenuated Mycobacterium tuberculosis (Mtb) vaccines (WHO recommendations BCG vaccines, 2013). Note that the traditional method of pellicle culture of BCG which was established in the pre-GMP era is difficult to match with cGMP guidelines.

At the end of this stage, an expression system and lab-scale process must be selected and used to produce pre-clinical R&D material. This process should, in theory, be based upon initial tests, literature and previous projects, and deliver the estimated quantity and quality of the TB vaccine candidate(s).

The process parameters are explored and optimised at lab scale. Examples of parameters for cell culture are the expansion of cells, temperature, oxygen, pH, stirrer speed, and concentration of metabolites and vaccine product. For down-stream process (DSP), examples of parameters are temperature, flow rate, salt concentration, or column height for ion-exchange column chromatography. Critical parameters for each step of the process must be identified, and optimum set points and ranges defined experimentally. Critical are the yield, purity and stability of intermediate products.

Caution must be taken with raw material used for culture media and buffers. Lots of raw material should be evaluated to control input and identify potential sources of variation to establish specifications that will support a stable process performance and product quality. The same is true for the cells (expression system/ biological source organism) that are used to produce antigen, which must be stable for multiple production runs. A risk assessment would be appropriate to estimate the potential impact of process variations on product quality (based on literature, other development projects, and regulatory requirements).

The product quantity (yield) is optimised assuming that the product quality will not change, which will be demonstrated later. The selected set-points for the different process steps is confirmed with product testing in animals.

The cost of production of the antigen and its vaccine formulation can be calculated, and, combined with yield and dose, provide information on 'Cost of Goods (CoG)’. In addition, a risk assessment is performed to indicate areas where there is insufficient control, which have to be addressed in following development steps to mitigate these risks.

References and links to Guidelines on process and production of vaccines can be found here.

The process is scaled up from lab-scale to pilot, at a scale calculated to generate sufficient material for toxicity studies and Phase 1 / 2 clinical studies.

MCB and WCB, or a combination of Master and Working Virus Seed Lot combined with a MCB and WCB of complementary host cells, are manufactured according to cGMP guidelines or equivalent quality level. After determining feasibility of the pilot-scale process performed with cells from the WCB, vials are sent for release testing.

Standardised production runs at pilot scale (pre-GMP runs) will confirm feasibility and reproducibility. Indeed, by demonstrating that repeated runs can be performed within certain ranges around the set-points for the relevant process parameters, the reproducibility of the process (or process steps) is established, as required for the pilot process according to GMP guidelines. The variation obtained in parameters which are considered critical should be within the ranges tested at lab scale. At the end of this Stage, the selected process now at pilot scale, can produce the right quantity of product with the correct quality (drug substance and contaminants) and, no further adjustments are required. Also, at this stage additional information on Cost of Goods could be gathered based on the standardised pilot process.

At the end of this stage, the process is fixed for manufacturing of Phase 1 GMP material. Later changes in the process can be required. It will have to be demonstrated that the implemented process changes do not affect the product quality attributes (product comparability). For TB vaccine candidates, later changes in the production process would likely need clinical bridging studies.

At the end of the pre-clinical development, a detailed report presents the production process with a rationale for each specified step and data. The standardised production process is also described in concept documents (standard procedures and production protocols). From this point, the process is fixed and can be transferred to Manufacturing and documents used for Tech transfer.

The MCB and WCB now have to be released for use in production runs according to cGMP. The Phase 1 material is produced according to cGMP and to pre-set criteria generated previously. The documents are approved by production experts and QA.

At this stage the estimate of Cost of Goods can be calculated and confirmed based on the standardised pilot process.

Further process development involving scale-up towards targeted market scale and process validation are the aims of Stage E.

A live attenuated TB vaccine is used as an example. The anticipated clinical dose is relatively low and, therefore, the scale could be 10 to 100L bioreactors, even for vaccines requiring multi-million doses because one mL of reactor volume could contain 10 (to the power of 6) doses. Minimal purification is required with low impact on yield, although loss can be caused by lyophilisation. A contrasting example is that of a non-live vaccine where the equivalent reactor volume (1 mL) is unlikely to produce a similar number of purified antigen doses as a live vaccine.

Assuming a down-stream process yield of 50%, and target dose of 10 µg, it means that for a multi-million dose requirement multiple 1000L reactor runs would be needed. In any event, the scale-up process for commercial batches should be defined at this stage.

Activities related to process development, scale up and validation described in Stage E continue during Stage F, up to process and scale for Phase 3 / commercial product.

The process must be validated at this stage. For validation, it is recommended to use a down-scale model, where the set-points and specified ranges are confirmed for each unit process by monitoring the Critical Quality Attributes (CQA). If this model cannot be used, the multiple runs performed at a larger scale must demonstrate that the product meets pre-set criteria (CQA as recorded in the Bill of Testing, Quality Target Product Profile) and that critical process parameters stay within the original ranges tested during preclinical development. Otherwise, rework is required to implement a revised process, possibly besides process development and validations, running the risk of requirement for bridging clinical studies.

The final developed process and scale are used for the manufacturing of Phase 3 material. It has reached a stage close to the volume required for manufacturing for the market. The 3-5 consecutive consistency runs performed at market scale (according to GMP guidelines) finalise the process validation work and will also generate the vaccine product to be used for Phase 3. Now is also the time to perform the calculation of Cost of Goods for the large scale production process.

Provided that all pre-set criteria for product and process are confirmed, meaning that no major out-of-specification issues occurred, the production process can be executed for routine manufacturing.

As for the production of product batches for Phase 3 (and final process validation), the initial vaccine product batches as commercial lots are produced.

An inventory and control system must be in place for the storage of biological source (master and working seeds), raw materials, in-process controls, reference standards, and purified product. The control system will have incorporated information of validation studies determining the stability of the different materials under the specified storage conditions. For the biological source stocks, the shelf life is often so long that it cannot be determined exactly and a new Working Seed Lot will be generated from the Master Seed lot when the vials of the existing Working Seed Lot have been (nearly) used for routine manufacturing. For the product reference standard, the shelf life will be generally coupled to the shelf life for the vaccine product.

For the routine production of vaccine product for the market, a production plant has to be available. For attenuated live strains of Mtb or BCG it is likely to be a dedicated production facility. For a subunit vaccine product, campaign-wise production after change-over in a multi-purpose facility could be considered, also linked to the required volumes.

All the resources required for sustainable delivery of a sufficient amount of product need to be secured at the correct quality level. This includes trained personnel, approved documentation, released seed stocks, raw materials and disposables, and validated production equipment. In addition, a system to effectively deal with deviations has to be in place (Corrective actions/Preventive actions for all types of failures, CAPA).

To support routine manufacturing the resources for long term QC activities need to be secured, at the required quality level (raw materials/ reagents, trained personnel, equipment, reference standards).