United States Patent:  6,051,239

Assignee:  Thomas Jefferson University (Philadelphia, PA)

Filed:  October 20, 1997

Compositions and methods of oral delivery of an antigen or therapeutic agent to the general circulation using a modified botulinum toxin which is capable of translocating from the gut to the general circulation but which is altered to be nontoxic are provided.

DETAILED DESCRIPTION OF THE INVENTION

One of the major challenges of modern medicine is the development of drugs that can be administered by the oral route. The development of oral peptide vaccines that evoke systemic immunity has proven to be especially problematic. Difficulties associated with the development of oral peptide vaccines include: degradation upon exposure to conditions of low pH and proteolytic enzymes found in the human gut; the antigenic domain of the agent which produces the illness being too large to allow for significant non-specific diffusion from the lumen of the gut to the general circulation; and an inability to design peptide vaccines that will bind exploitatively to receptors in the gut and undergo active transport to the general circulation. Despite these difficulties, considerable effort is being invested in the search for oral vaccines. For example, the concept of using engineered food such as a potato or a banana as a vector for widescale vaccination has recently been proposed. However, engineering the antigenic peptide into a food which is then ingested does not overcome these difficulties. Accordingly, there is a need for drug delivery vehicles which will reliably and reproducibly translocate an antigenic peptide or other therapeutic agent from the gut to the general circulation.

The present invention provides a modified botulinum toxin which can be used as an oral delivery vehicle for antigenic peptides including, but not limited to, botulinum toxin and other therapeutic agents to the general circulation. It has now been found that botulinum toxin translocates from the gut to the general circulation by binding to serospecific receptors on the mucosal side of polarized gut cells grown in a monolayer. Bound toxin is actively transported across the cells and delivered intact and unmodified on the serosal side of the monolayers. It has been suggested that auxiliary proteins such as hemagglutinin, which is a component of the non-covalent complex of proteins including the botulinum toxin which is released by Clostridium, may mediate binding and transport of the toxin across the gut wall. However, experiments performed with a recombinant form of the holotoxin now demonstrate that the botulinum toxin itself possesses the binding domain that recognizes receptors on the surface of gut cells. Further, it has now been demonstrated that modifications can be made to the light chain of the toxin to render it nontoxic without altering the capability of the protein to translocate from the gut to the general circulation. Accordingly, for the purposes of the present invention, by "modified botulinum toxin" is meant a botulinum toxin which maintains its capability of translocating from the gut to the general circulation but which is nontoxic. Alterations which will render the botulinum toxin nontoxic include mutations to the amino acid sequence of the light chain and deletion of the light chain or portions thereof. In a preferred embodiment, mutations are made to the zinc binding motif or the substrate binding motif of the light chain. For the purposes of this invention, by "nontoxic" it is meant that exposure of the cholinergic nerve endings to the modified botulinum toxin does not result in blockade of transmitter release in the nerve endings and paralysis. The effects of alterations rendering the botulinum toxin nontoxic on the ability of the toxin to translocate from the gut to the general circulation can be routinely performed in accordance with the teachings provided herein so that one of skill may identify modified botulinum toxins of the present invention. Included within this definition of modified botulinum toxins are botulinum toxins which further comprise a selected antigen for a protein other than botulinum toxin or a therapeutic agent.

For example, compositions were prepared comprising a botulinum neurotoxin in which the zinc binding motif of the light chain of the holotoxin was inactivated. The modified toxin is nontoxic because the holotoxin does not retain the ability to cause neuromuscular blockade, but the modification to the light chain does not adversely affect the ability of the remainder of the toxin molecule to escape the lumen of the gut into the general circulation. In this preferred embodiment at least three of the amino acids comprising the zinc binding motif of the light chain were modified. Specifically, the amino acids His (at position 229), Glu (at position 230), and His (at position 233) of the native sequence were substituted with amino acids Gly, Thr and Asn, respectively, resulting in SEQ ID NO: 1. The nucleic acid sequence encoding this modified botulinum toxin is depicted as SEQ ID NO: 2. These amino acid substitutions eliminated the ability of the holotoxin to bind catalytic zinc or other divalent cations.

Experiments have also been performed demonstrating that unnicked or single chain botulinum toxin also binds and is transported across the gut wall. Accordingly, modified botulinum toxins of the present invention also include compositions wherein the nicking site has been eliminated.

The biological activity of a modified botulinum toxin of the present invention was determined via an in vivo toxicity test, in vitro activity on the mouse phrenic nerve-hemidiaphragm preparation, and enzymatic activity in crude synaptosome preparations. For these experiments, the modified botulinum toxin, referred to herein as modified recombinant or modified rBoNT/C, was generated from botulinum toxin serotype C using site-directed mutagenesis to inactivate the zinc binding motif from the light chain of the holotoxin that is essential for endoprotease activity. However, other methods of peptide synthesis including, but not limited to, biochemical techniques, such as enzymatically cutting a peptide and cross linking the resulting fragments which are performed routinely by those of skill in the art can also be used. Further, given the structural and functional similarities of the botulism serotypes, one of skill could routinely prepare modified botulinum toxins from serotypes other than botulinum serotype C. For example, all serotypes of botulinum toxins are synthesized as relatively inactive precursors with molecular weights of approximately 150,000. In each case, the precursors must be "nicked" by a protease to generate a dichain molecule having a heavy chain (100,000 kDa) linked by a disulfide bond to a light chain (50,000 kDa) . Every serotype of botulinum toxin acts preferentially on cholinergic nerve endings to block transmitter release, with the heavy chain acting principally as a tissue-targeting domain to direct the toxin to cholinergic nerve endings, and the light chain acting inside the nerve ending to block transmitter release. It is the light chain of every serotype that acts as a zinc-dependent metalloendoprotease to cleave one or more members of a family of polypeptides that is essential for transmitter release. In every serotype, there is a zinc binding motif, His-Glu-X-X-His (SEQ ID NO: 3) that is essential for enzymatic activity. Modification of the binding motif invariably causes loss of enzymatic activity. Further, alignment of the nucleic acid and amino acid sequences for a portion of each serotype encompassing the region of the zinc binding motif demonstrates a high degree of sequence identity in the regions adjacent to and comprising the zinc binding motif. Thus, examples using botulinum serotype C are representative of the entire class.

In vivo toxicity testing of modified rBoNT/C holotoxin demonstrated that the modified botulinum toxin with mutations in the zinc binding motif produced no acute toxicity in mice during a 16 week monitoring period following administration, even at high doses (10 .mu.g per animal, i.p.). No apparent neurotoxicity or other obvious harmful effects were observed in any of the animals. In contrast, mice injected with 100 ng i.p. native BoNT/C died within 2 to 2.5 hours of injection.

The in vitro toxicity of modified BoNT/C holotoxin was also compared with that of native BoNT/C in mouse phrenic nerve-hemidiaphragm preparations. It was found that the addition of the modified botulinum toxin to phrenic nerve-hemidiaphragm preparations did not produce neuromuscular blockade (1x10^-10 M; n=4). By contrast, addition of native BoNT/C (1x10^-12 M; n=8) invariably produced paralysis of transmission (mean.+-.S.E.M.=152.+-.17 min).

The ability of this modified botulinum toxin to evoke an immune response was also tested after oral (p.o.) administration and subcutaneous (s.c.) injection. As determined by immunoblot analysis, both p.o. and s.c. administration of modified rBoNT/C holotoxin evoked systemic antibody production. Accordingly, the modified botulinum toxin of the present invention maintained its ability survive transit through the gut and to undergo active translocation out of the gut. This is further evidenced by the finding that s.c. administration of a non-homogeneous preparation of the modified botulinum toxin, which contained small amounts of unrelated proteins, is able to evoke an immune response against these unrelated proteins, while p.o. administration evoked antibody only against the modified botulinum toxin.

The protective effect of the antibodies elicited by p.o. and s.c. administration of the modified botulinum toxin was then demonstrated in both serum neutralization and in vivo toxicity tests. Regardless of the route of administration, serum from animals immunized with modified botulinum toxin inactivated a large dose (.about.10,000 LD₅₀) of native BoNT/C. Similarly, in in vivo toxicity tests, immunization with the modified botulinum toxin by either the p.o. or s.c. route produced a dramatic reduction in the potency of a subsequent injection of native toxin. Animals given the modified botulinum toxin by the oral route of administration had detectable antibodies in serum for at least three months. Further, animals that received the modified botulinum toxin either p.o. or s.c. were protected against native BoNT/C challenge three months after the third booster.

Accordingly, results from these experiments demonstrate that a modified botulinum toxin can be constructed in accordance with the teachings provided herein that is nontoxic but which retains the ability to translocate from the gut to the general circulation and to evoke protective antibodies. Further, compositions comprising a modified botulinum toxin of the present invention are clearly effective as oral vaccines against botulism in animals.

In addition, because the modified botulinum toxins of the present invention retain their ability to translocate from the gut and to be delivered intact to the general circulation, these modified botulinum toxins can be used as delivery vehicles for oral administration of antigens to proteins other than botulinum toxin and therapeutic agents to the general circulation. There are various ways in which the modified botulinum toxin could be used as a carrier for oral vaccines. For example, because the inactivation of the zinc binding motif of the light chain does not adversely affect the toxin's ability to translocate out of the gut, the zinc binding motif of the native botulinum toxin can be replaced with a selected antigen for a different protein, i.e. a protein other than botulism, to produce an oral vaccine against this different protein. Alternatively, well known techniques of protein chemistry and molecular biology can be used to attach the selected antigen or a portion thereof to a modified botulinum toxin. The resulting modified botulinum toxin would not only be nontoxic, but also retain its ability to translocate from the gut to the general circulation so that the selected antigen, when administered orally, would reach the general circulation to evoke a systemic immune response against the protein. Examples of vaccines which could be administered orally with the modified botulinum toxin include, but are not limited to, vaccines for Bacille Calmette-Guerin, cholera, diphtheria, hepatitis B, measles, meningitis, mumps, pertussis, plague, polio, rabies, rubella, tetanus, typhoid, and yellow fever. The oral vaccine can be administered individually or in combination, such as for DTP (diphtheria, tetanus, pertussis). The ability to deliver an oral vaccine is especially important for areas in which medical personnel are not readily available. Moreover, an oral vaccine of the present invention would represent an important economic advantage in addition to diminishing the need for skilled personnel as it would eliminate costs associated with syringes used for injection and/or for the disposal of used syringes.

Formulations of oral vaccines of the present invention preferably comprise the modified botulinum toxin in a pharmacologically acceptable carrier, such as sterile physiological saline, sterile saline with 0.1% gelatin, or sterile saline with 1.0 mg/ml bovine serum albumin. Alternatively, the modified botulinum toxin of the present invention can be genetically engineered into a plant so that food produced by the plant such as a potato or a banana can serve as a vector for widescale vaccination. Methods of genetically engineering plants to express a foreign peptide are well known in the art as exemplified by PCT/US96/09558, filed Jun. 6, 1996.

The modified botulinum toxins of the present invention are also useful in the construction of chimeric oral therapeutics. In this embodiment, a therapeutic agent can be linked to modified botulinum toxin to yield two broad groups of orally administered molecules: (1) new drugs with biologically stable linkages, and (2) conjugate prodrugs having biologically or chemically unstable linkages, which dissociate from the carrier upon reaching the blood. Examples of chimeric therapeutic techniques are described generally by Lautenslager, G. T. and Simpson, L. L., "Chimeric Molecules Constructed with Endogenous Substances," Advances in Molecular and Cell Biology, Vol. 9, pp. 233-262, JAI Press, Inc. (1994). For example, a therapeutic peptide could be attached to a modified botulinum toxin, thus creating an agent which possesses the characteristics of the substituent yet is capable of being administered orally. One example would be the creation of an orally administered thrombolytic agent. A fusion protein constructed by combining P-selectin and tissue plasminogen activator (TPA) is a promising chimera which expresses thrombolytic activity and targets to the thrombi. This chimera must be introduced into the blood stream. However, using either molecular biology or protein chemistry, this `first order` chimeric molecule could be attached to a modified botulinum toxin of the present invention to create a higher order chimera which possesses the added advantage of being delivered to the general circulation by oral administration. Another example is in the design of an orally administered anti-neoplastic drug. Various antineoplastic drugs which exploit the cytotoxic properties of one molecule, fused to a portion of another which functions to specifically target the toxin have been disclosed. A more recent example employs the amino-terminus of Pseudomonas exotoxin (PE) fused to epidermal growth factor (EGF), resulting in chimera EGF-PE which can be used as a cytotoxic agent towards EGF-receptor-bearing cancer cells. Linkage of this chimera to a modified botulinum toxin of the present invention would result in creation of a higher order chimera which can be administered orally.

The general concepts for use of a modified botulinum toxin as a carrier for vaccines or other therapeutic agents are the same for human and for non-human animals, with one exception. All serotypes of botulinum toxin are not likely to be equally efficacious as carriers for drugs in all species. Clinical evidence suggests that humans are especially sensitive to the effects of serotypes A, B, and E. This may relate to the efficiency with which these three serotypes are absorbed from the gastrointestinal system. Thus, serotypes A, B, and E would be preferred carriers of therapeutic agents for humans.

On the contrary, most non-human animals are particularly sensitive to serotype C. This suggests that as to veterinary medicine, the preferred carrier of therapeutic agents for non-human animal use would be serotype C. Examples of animal vaccines which could be administered orally with the modified botulinum toxin include, but are not limited to, ones for adenovirus type 2, Bordetella bronchispetica, botulism, calicivirus, Chlamydia psittaci, clostridial diseases, such as Clostridium Perfringens type C, coronaviruses, distemper, equine encephalomyelitis, Escherichia coli, feline infectious peritonitis, feline leukemia virus, feline panleukopenia, hepatitis, leptospirosis, parainfluenza virus, parvoviruses, rabies, rhinotracheitis virus, and tetanus.

Claim 1 of 2 Claims

1. A modified botulinum toxin comprising a botulinum toxin capable of translocating from the gut to the general circulation and a selected antigen, wherein said botulinum toxin is altered to be nontoxic by mutating or deleting amino acids in the light chain of the botulinum toxin.

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