The present invention relates to a pharmaceutical composition comprising at least one mRNA comprising at least one coding region for at least one antigen from a tumour, in combination with an aqueous solvent and preferably a cytokine, e.g. GM-CSF, and a process for the preparation of the pharmaceutical composition. The pharmaceutical composition according to the invention is used in particular for therapy and/or prophylaxis against cancer.
Gene therapy and genetic vaccination are molecular medicine methods which, when used in the therapy and prevention of diseases, will have considerable effects on medical practice. Both methods are based on the introduction of nucleic acids into cells or into tissues of the patient and on subsequent processing of the information coded by the nucleic acids introduced, i.e. expression of the desired polypeptides.
The conventional procedure of methods of gene therapy and of genetic vaccination to date is the use of DNA to insert the required genetic information into the cell. Various methods for introducing DNA into cells have been developed in this connection, such as e.g. calcium phosphate transfection, polyprene transfection, protoplast fusion, electroporation, microinjection and lipofection, whereas lipofection in particular having emerged as a suitable method.
A further method which has been proposed in particular in the case of genetic vaccination methods is the use of DNA viruses as DNA vehicles. Such viruses have the advantage that because of their infectious properties a very high transfection rate can be achieved. The viruses used are genetically modified, so that no functional infectious particles are formed in the transfected cell. In spite of this safety precaution, however, a certain risk of uncontrolled propagation of the genes having a gene therapy action and the viral genes introduced cannot be ruled out because of possible recombination events.
The DNA introduced into the cell is conventionally integrated into the genome of the transfected cell to a certain extent. On the one hand this phenomenon can exert a desired effect, since a long-lasting action of the DNA introduced can thereby be achieved. On the other hand, the integration into the genome results in a substantial risk of gene therapy. Thus e.g. the DNA introduced may be inserted into an intact gene, which represents a mutation which interferes or even completely switches off the function of the endogenous gene. On the one hand enzyme systems which are essential for the cell may be switched off by such integration events, and on the other hand there is also the danger of a transformation of the cell modified in this way into a degenerated state if a gene which is decisive for regulation of cell growth is modified by the integration of the foreign DNA. A risk of the development of cancer therefore cannot be ruled out when using DNA viruses as gene therapeutics and vaccines. In this connection it is also to be noted that for effective expression of the genes introduced into the cell, the corresponding DNA vehicles contain a strong promoter, e.g. the viral CMV promoter. Integration of such promoters into the genome of the treated cell can lead to undesirable changes in the regulation of gene expression in the cell.
A further disadvantage of the use of DNA as gene therapeutics and vaccines is the induction of pathogenic anti-DNA antibodies in the patient, causing a possibly fatal immune response.
In contrast to DNA, the use of RNA as a gene therapeutic or vaccine is to be classified as substantially safer. In particular, RNA does not involve the risk of being integrated into the genome of the transfected cell in a stable manner. Furthermore, no viral sequences, such as promoters, are necessary for effective transcription. Moreover, RNA is degraded considerably more easily in vivo. Apparently because of the relatively short half-life of RNA in the blood circulation compared with DNA, no anti-RNA antibodies have been detected to date. RNA can therefore be regarded as the molecule of choice for molecular medicine therapy methods.
Nevertheless, medical methods based on RNA expression systems still require a solution to some fundamental problems before they are used more widely. One of the problems of using RNA is reliable cell- or tissue-specific efficient transfer of the nucleic acid. Since RNA usually proves to be very unstable in solution, it has not hitherto been possible, or has been possible only in a very inefficient manner, to use RNA as a therapeutic or vaccine by the conventional methods which are used with DNA.
RNA-degrading enzymes, so-called RNAases (ribonucleases), are responsible for the instability. Even the smallest impurities of ribonucleases are sufficient to degrade RNA in solution completely. The natural degradation of mRNA in the cytoplasm of cells is very finely regulated. Several mechanisms are known in this respect. Thus, the terminal structure is of decisive importance for a functional mRNA. At the 5′-end is the so-called “cap structure” (a modified guanosine nucleotide), and at the 3′-end a sequence of up to 200 adenosine nucleotides (the so-called poly-A tail). The RNA is recognized as mRNA and the degradation is regulated via these structures. Moreover, there are further processes which stabilize or destabilize RNA. Many of these processes are still unknown, but an interaction between the RNA and proteins often appears to be decisive for this. For example, an “mRNA surveillance system” has recently been described (Hellerin and Parker, Annu. Rev. Genet. 1999, 33: 229 to 260), in which incomplete or nonsense mRNA is recognized by certain feedback protein interactions in the cytosol and is rendered accessible to degradation, the majority of these processes being performed by exonucleases.
Some measures for increasing the stability of RNA and thereby rendering possible its use as a gene therapeutic or RNA vaccine have been proposed in the prior art.
To solve the abovementioned problems of the instability of RNA ex vivo, EP-A-1083232 proposes a process for introduction of RNA, in particular mRNA, into cells and organisms, in which the RNA is in the form of a complex with a cationic peptide or protein.
WO 99/14346 describes further processes for stabilizing mRNA. In particular, modifications of the mRNA which stabilize the mRNA species against the degradation by RNases are proposed. Such modifications concern on the one hand stabilization by sequence modifications, in particular reduction of the C and/or U content by base elimination or base substitution. On the other hand, chemical modifications, in particular the use of nucleotide analogues, and 5′- and 3′-blocking groups, an increased length of the poly-A tail and complexing of the mRNA with stabilizing agents and combinations of the measures mentioned, are proposed.
The U.S. Pat. No. 5,580,859 and U.S. Pat. No. 6,214,804 disclose, inter alia, mRNA vaccines and therapeutics in the context of “transient gene therapy” (TGT). Various measures for increasing the translation efficiency and the mRNA stability based above all on untranslated sequence regions are described.
Bieler and Wagner (in: Schleef (ed.), Plasmids for Therapy and Vaccination, chapter 9, pages 147 to 168, Wiley-VCH, Weinheim, 2001) report on the use of synthetic genes in connection with gene therapy methods using DNA vaccines and lentiviral vectors. The construction of a synthetic gag gene derived from HIV-1, in which the codons were modified (alternative codon usage) compared with the wild-type sequence such that they corresponded to the use of codons which are to be found in highly expressed mammalian genes, is described. By this means, the A/T content in particular was reduced compared with the wild-type sequence. The authors find in particular an increased expression rate of the synthetic gag gene in transfected cells. Furthermore, in mice an increased formation of antibodies against the gag protein was observed in mice immunized with the synthetic DNA construct, and also an increased cytokine release in vitro in transfected spleen cells of mice. Finally, an induction of a cytotoxic immune response was to be found in mice immunized with the gag expression plasmid. The authors of this article attribute the improved properties of their DNA vaccine substantially to a change, caused by the optimized codon usage, to the nucleo-cytoplasmic transporation of the mRNA expressed by the DNA vaccine. In contrast, the authors consider the effect of the modified codon usage on the translation efficiency to be low.