Aptamers, are oligonucleotides, which can be synthetic or natural, that bind to a particular target molecule, such as a protein or metabolite. Typically, the binding is through interactions other than classic Watson-Crick base pairing.
Aptamers represent a promising class of therapeutic agents currently in pre-clinical and clinical development. Like biologics, e.g., peptides or monoclonal antibodies, aptamers are capable of binding specifically to molecular targets and, through binding, inhibiting target function. A typical aptamer is 10-15 kDa in size (i.e., 30-45 nucleotides), binds its target with sub-nanomolar affinity, and discriminates among closely related targets (e.g., will typically not bind other proteins from the same gene family) (Griffin, et al. (1993), Gene 137(1): 25-31; Jenison, et al. (1998), Antisense Nucleic Acid Drug Dev. 8(4): 265-79; Bell, et al. (1999), In Vitro Cell. Dev. Biol. Anim. 35(9): 533-42; Watson, et al. (2000), Antisense Nucleic Acid Drug Dev. 10(2): 63-75; Daniels, et al. (2002), Anal. Biochem. 305(2): 214-26; Chen, et al. (2003), Proc. Natl. Acad. Sci. U.S.A. 100(16): 9226-31; Khati, et al. (2003), J. Virol. 77(23): 12692-8; Vaish, et al. (2003), Biochemistry 42(29): 8842-51).
Aptamers can be created by an entirely in vitro selection process (Systematic Evaluation of Ligands by Experimental Enrichment, i.e., SELEX™) from libraries of random sequence oligonucleotides as described in U.S. Pat. Nos. 5,475,096 and 5,270,163. Aptamers have been generated against numerous proteins of therapeutic interest, including growth factors, enzymes, immunoglobulins, and receptors (Ellington and Szostak (1990), Nature 346(6287): 818-22; Tuerk and Gold (1990), Science 249(4968): 505-510).
Aptamers have a number of attractive characteristics for use as therapeutics. In addition to high target affinity and specificity, aptamers have shown little or no toxicity or immunogenicity in standard assays (Wlotzka, et al. (2002), Proc. Natl. Acad. Sci. U.S.A. 99(13): 8898-902). Indeed, several therapeutic aptamers have been optimized and advanced through varying stages of pre-clinical development, including pharmacokinetic analysis, characterization of biological efficacy in cellular and animal disease models, and preliminary safety pharmacology assessment (Reyderman and Stavchansky (1998), Pharmaceutical Research 15(6): 904-10; Tucker et al., (1999), J. Chromatography B. 732: 203-212; Watson, et al. (2000), Antisense Nucleic Acid Drug Dev. 10(2): 63-75).
It is important that the pharmacokinetic properties for all oligonucleotide-based therapeutics, including aptamers, be tailored to match the desired pharmaceutical application. While aptamers directed against extracellular targets do not suffer from difficulties associated with intracellular delivery (as is the case with antisense and RNAi-based therapeutics), the aptamer must be distributed to target organs and tissues, and remain in the body (unmodified) for a period of time consistent with the desired dosing regimen. Early work on nucleic acid-based therapeutics has shown that, while unmodified oligonucleotides are degraded rapidly by nuclease digestion, protective modifications at the 2′-position of the sugar, and use of inverted terminal cap structures, e.g., [3′-3′dT], dramatically improve nucleic acid stability in vitro and in vivo (Green, et al. (1995), Chem. Biol. 2(10): 683-95; Jellinek, et al. (1995), Biochemistry 34(36): 11363-72; Ruckman, et al. (1998), J. Biol. Chem. 273(32): 20556-67; Uhlmann, et al. (2000), Methods Enzymol. 313: 268-84). In some SELEX selections (i.e., SELEX experiments or SELEX ions), starting pools of nucleic acids from which aptamers are selected are typically pre-stabilized by chemical modification, for example by incorporation of 2′-fluoropyrimidine (2′-F) substituted nucleotides, to enhance resistance of aptamers against nuclease attack. Aptamers incorporating 2′-O-methylpurine (2′-OMe purine) substituted nucleotides have also been developed through post-SELEX modification steps or, more recently, by enabling synthesis of 2′-OMe-containing random sequence libraries as an integral component of the SELEX process itself.
In addition to clearance by nucleases, oligonucleotide therapeutics are subject to elimination via renal filtration. As such, a nuclease-resistant oligonucleotide administered intravenously exhibits an in vivo half-life of <10 min, unless filtration can be blocked. This can be accomplished by either facilitating rapid distribution out of the blood stream into tissues or by increasing the apparent molecular weight of the oligonucleotide above the effective size cut-off for the glomerulus. Conjugation to a PEG polymer (“PEGylation”) can dramatically lengthen residence times of aptamers in circulation, thereby decreasing dosing frequency and enhancing effectiveness against targets. Previous work in animals has examined the plasma pharmacokinetic properties of PEG-conjugated aptamers (Reyderman and Stavchansky (1998), Pharmaceutical Research 15(6): 904-10; Watson, et al. (2000), Antisense Nucleic Acid Drug Dev. 10(2): 63-75)). Determining the extravasation of an aptamer therapeutic, including aptamer therapeutics conjugated to a modifying moiety or containing modified nucleotides and, in particular, determining the potential of aptamers or their modified forms to access diseased tissues (for example, sites of inflammation, or the interior of tumors) define the spectrum of therapeutic opportunities for aptamer intervention.
Typically, therapeutic aptamers are administered by injection, for example, by subcutaneous injection. Accordingly, the aptamer must be dissolved in a liquid vehicle for administration. The relatively high molecular weight of aptamers, and in particular aptamers that have been derivatized, for example by PEGylation, however, often makes it difficult to obtain a pharmaceutical composition wherein the aptamer is dissolved in a pharmaceutically acceptable solvent at a sufficient concentration to provide a pharmaceutical composition that is clinically useful for administration to an animal.
U.S. published application no. 2005/0175708 discloses a composition of matter that permits the sustained delivery of aptamers to a mammal. The aptamers are administered as microspheres that permit sustained release of the aptamers to the site of interest so that the aptamers can exert their biological activity over a prolonged period of time. The aptamers, can be anti-VEGF aptamers.
P. Burmeister et al., (2004), Chemistry and Biology: 15, 25-33 disclose a method for generating a 2′-O-methyl aptamer (ARC245) that binds to vascular endothelial growth factor, which exhibits good stability.
Accordingly, there is a need in the art for improved pharmaceutical compositions, wherein the therapeutic agent is an aptamer. In particular, there is a need for pharmaceutical composition wherein the aptamer can be dissolved in a pharmaceutically acceptable solvent at a sufficient concentration to provide a pharmaceutical composition that is clinically useful for administration to an animal. The present invention addresses this as well as other needs.
Citation of any reference in this application is not to be construed as an admission that such reference is prior art to the present application.