Meeting on Combination Vaccines

Copenhagen, 23 January 2002

Copenhagen, 23 January 2002

The scientists listed below were invited

The group of four - Klavs Berzins, Giampietro Corradin, Pierre Druilhe, Adrian Hill prepared the consensus document, and comments were invited from the eight scientists listed below. All comments except one have been incorporated in the body of the document. One major comment appears as a separate paragraph at the end of the document.

Pedro Alonso
David Arnot
Peter Kremsner
Adrian Luty
Robert Sauerwein
Geoffrey Targett
Alan Thomas
Mats Wahlgren

The meeting – an informal preparatory meeting – was called to debate the rationale for combination vaccines with the aim of preparing a working document to enable EMVI to launch a call for mono or/and multi-components for combination vaccines.

Combination or multi-component vaccines are in this context defined as vaccines directed against one disease, malaria, and exclude combined vaccines against several diseases such as diphtheria, tetanus, polio etc.

The participants briefly discussed whether combination vaccines should be considered, and agreed that there were valid reasons for discussing the issue, and further agreed that the forum, format and scope were acceptable.

Discussion

The group agreed that, based on text books and review articles, there is a widely accepted dogma that "at the end of the day we will have a multi-component malaria vaccine". There is, however, no solid scientific evidence to suggest that only combination or multi-component vaccines should be pursued.

A brief discussion took place concerning terminology; what is actually meant by combination vaccines. There are a wide variety of rationales concerning combination vaccines, and it was agreed to define a combination vaccine as having components from more than one antigen. It was felt necessary to be absolutely precise as to the various categories, and a non-exhaustive list of combination vaccines was drawn up.

1. Multi-epitope combination: Different epitopes selected from different antigens, combined together – whether from the same gene or from several genes, but usually for combination vaccines it is understood that the multi-epitope combination is derived from at least two genes.
2. Multi-gene or multi-antigenic combinations 10.0pt;font-family: Arial"> (differing from 1): Where two antigens, such as two full-length proteins, are combined. The definition also covers parts of genes, which define several epitopes rather than a single epitope, by combining different recombinant fragments derived from large proteins.
3. Single-stage combinations - Multi-component vaccines derived from the same stage: Combination of proteins or plasmids from genes expressed in a single stage of the life cycle of the malaria parasite.
4. Multi-stage combinations: Proteins or plasmids corresponding to antigens expressed in different successive stages of the parasite, i.e. the combination of a liver stage vaccine with a blood stage vaccine, or a blood stage vaccine with a gametocyte stage vaccine, etc. These combinations are based on a different rationale than 3), in that they are aimed at stopping the parasite at different stages in its life cycle.
5. Simultaneous immunisation: Different epitopes or different proteins or different genes are combined in one construct and injected simultaneously. Alternatively, separate genes or separate proteins are combined with or without an adjuvant and injected simultaneously.
6. Combinations based on knowledge: Combinations where each epitope or each protein is carefully selected based on a proven biological activity, i.e. at least a defined immune response, preferentially an effect on parasite replication or invasion, and differing from:
7. Random combinations: Several genes or several proteins are combined, without a precise rationale for selection of the molecules - other than they "worked" in an animal model - but based on an idea that a combination will be beneficial. A good example is the Must-Do 5.1., 9.1., 15.1. Many other experimental vaccines have also been tested with randomly chosen candidates, generally those antigens available.

The group then went on to discuss the merit of the bottom up approach, in which each component is validated separately before mixing, as opposed to the top down approach where individual components are chosen on the basis of a hypothesis, and not necessarily on their experimental validation. Both approaches were considered as having their own merits and advantages.

When considering combination vaccines, it is also important to take into account "arithmetic" problems and psychological or communication problems.

All published potential malaria vaccine candidates, irrespective of their degree of validation and development, count for less than 1% of the proteins that the P. falciparum genome codes for. Nevertheless, to validate and develop the individual antigens in a variety of delivery systems and adjuvants is a formidable task.

The "arithmetic" problem can be illustrated by calculating formulations derived from the most studied gene, the circumsporozoite protein, which has been delivered by at least the following antigen delivery systems: synthetic peptides, recombinant proteins, long synthetic peptides, multiple-antigen and peptides (octopus), pox virus recombinants, genetic immunisation with a gene cloned in a plasmid, viral particles such as RTSS, attenuated vaccinia, prime-boost combination such as DNA followed by MVA, combined peptides, combined genetic immunisations etc. Each of these formulations is used in different doses, different delivery systems and for the most part with different adjuvants. Thus, from the above list, there is a large number of combination possibilities for a single protein alone. Once such range of possibilities, derived from a single gene, is combined - in combination vaccines - with at least one, or several others, it stands to reason that for just two genes, the number of combinations, relying on different doses, different delivery systems, different schemes, different numbers of injections is already quite large.

When combinations are made with several different genes, it is evident that the possible number of combinations is almost unlimited.

This raises unavoidable serious practical problems concerning numbers of volunteers involved, or funding for such a large number of possible combinations. Psychological and communication problems arise, when after 5, 10, 15, 20 clinical trials, there comes a time when agencies funding research may question how many clinical trials are needed to define a formulation which is effective, although this has not been a problem for HIV vaccines.

Many different combinations can be envisaged, and the main problem is likely to be, not when to start but rather when to stop.

The group went on to discuss whether any animal model could convincingly show that a combination of antigens was more protective than single antigens. It was felt that there was little published evidence on this aspect. Although there is evidence that antigens can act synergistically, as well as additively, from literature there is also evidence that a combination of malaria antigens can decrease the immunogenicity of the individual components, e.g. the combination of CS and LSA1 and the combination of RTSS and TRAP.

The group agreed unanimously that:

1. combination vaccines should preferentially be further developed when individual components have shown some degree of biological effect – ideally protective effect (in vitro? – in vivo?). As a minimum single components should be evaluated in clinical trials prior to or simultaneously with combination vaccines,
2. combination formulations should preferably demonstrate a synergistic or additive immunogenicity effect over and above the individual components’ immunogenicity, and as a minimum perform as well as the individual components on their own.
3. the combination should not lead to an antagonistic effect, i.e. a major decrease in the individual components’ immunogenicity would be a concern.

There was general agreement that combination vaccines should be based on knowledge of T, B or CTL epitopes, whether originating from the same or different genes. The known or presumed effector mechanism would serve as guidance for establishing priorities for the combination. This led to the notion that for practical purposes it would be preferable to concentrate on combination vaccines with components from the same stage of the parasite’s life-cycle, or combinations where the in vitro outcome measure could be compared. The view that combinations of epitopes from different stages can be developed simultaneously with combination vaccines from the same stage was also advocated, provided that the criteria for multi-stage combination and single stage combinations were identical, i.e. proof of biological effect was established.

It was emphasised that the inclusion of proper T-cell epitopes to ensure long lived memory response was important, and that this memory response should ideally be activated by natural infection to quickly induce protective levels.

The group debated the criteria for, and at what stage in development advancement should take place of combination vaccines to field testing, i.e. Phase I and II in a malaria endemic area. It was generally agreed that general criteria could not be applied as delivery systems, adjuvants and antigens vary considerably. Decisions on field testing would therefore have to be taken on a case by case basis, based on results from Phase I and II in European volunteers.

Whereas protective efficacy for pre-erythrocytic vaccines can be predicted to a certain degree from laboratory challenge trials, clinical trials in Africa have shown that certain vaccine immunogenicity is better than that obtained in Phase I/IIa studies in non-endemic areas (parasite priming?). On the other hand immunogenicity can also be the same or slightly less, as is the case for the RTSS vaccine. It thus seems prudent to advance Phase I trials of individual components of combination vaccines in endemic areas - provided that biological activity is demonstrated in trials in non-endemic areas - simultaneously with clinical trials of combination vaccines in non-endemic areas, in order to investigate whether safety and immune response alters when components are combined.

It was advocated that there was a great deal to be learned from clinical trials about immune responses, and therefore whenever possible (provided funds are available) to initiate a variety of small scale exploratory clinical Phase I, and where appropriate Phase II trials in Europe. There was also the feeling that caution should be exercised when advocating that a strong immune response is likely to induce better protection than a weak response, e.g. the irradiated sporozoite vaccine, which seems to elicit a weak immune response to specific antigens, offers very strong protection (see also above).

There was a consensus that further advancement and clinical testing of combination vaccines would require:

• Proof of biological activity of individual components from clinical trials, but that
• pre-clinical data, if very convincing, could occasionally serve as justification for development and testing of a combination vaccine, plus
• for any combination vaccine, it must be assumed that the means to evaluate the utility of each component is available.

It was advocated that the results of clinical trials should be carefully compared with pre-clinical data, in order to validate pre-clinical animal and in vitro models’ predictive value.

The group further felt it useful to stress that there is a need for additional clinical trial centres in Europe capable of conducting the numerous clinical studies needed, particularly challenge studies. Currently, only Oxford and Nijmegen have this capability, and they are collaborating on development of standardised monitoring. Information was provided that the group in Lausanne is working on establishing a facility for challenge trials. A debate on whether to establish a facility in other locations has also taken place.

In conclusion it is likely that combination vaccines with well documented components will be advantageous, and will therefore have to be evaluated, regardless of whether the components stem from the same parasite stage or from multiple-stages. However, to date there is no definite proof of this concept’s superiority.

There is a need to conduct many clinical trials in order to understand and evaluate immune responses, (besides the safety issue), and there is a need to increase challenge capacity in Europe.

Addendum

Comment:
Thank you for your report, which is a welcome start for a co-ordinated policy towards malaria vaccine development. It also embarks on a number of issues that were addressed in Groesbeek (see report). I support the general outline of thinking, as well as the approach chosen, i.e. proof of principle, or at least some defined effect of single components before combining.

The role of animal models, in particular monkeys, is not specifically discussed, but rather mentioned at the site. Particularly when it comes to simultaneous comparison of endemic vs. non-endemic trials (I like the idea), I also think that this, where possible, should be related to monkey models, in particular rhesus (best model?), assuming that chimp is not a practical option furthermore.

In addition, the complex issues of combining antigens, which is fraught with difficulties and potential failures, should be carefully considered. My strong opinion that testing of components, which are still at the individual stage, should always be in the perspective of a future partner. This means that adjuvant and immunisation schedule should be standardised and the sponsor, funding a number of trials and products, could play an instrumental role in this. I can see that this is a controversial remark, but this is how I see it. Resources are limited, and we cannot afford to start all over again time after time.

My last remark is that I would combine already as much as possible before going to Africa with the condition that protection trials (monkey or human in non-endemic) are validated. This needs to be done first and foremost.