There is increasing interest in the use of aptamers for the development of therapeutics. However, as oligonucleotides, aptamers are susceptible to nuclease degradation; poor serum stability is likely to negatively affect in vivo function. Modified nucleotides have been used to thwart nuclease degradation. However, few studies report the serum stability of selected aptamers. In this study, we examined the effect of various chemical modifications (2'-deoxy, 2'-hydroxyl, 2'-fluoro, and 2'-O-methyl) on the stability of a control oligonucleotide sequence following incubation in frozen human, fresh mouse, and fresh human serum. We also assessed the effect of the 3' inverted dT cap on stability. Surprisingly, we found that fYrR (2'-fluoro RNA) is only roughly as stable as DNA (2'-deoxy). Interestingly, the inclusion of a 3' inverted dT cap had only a modest effect on serum stability, if any. In one instance, the addition of a 3' inverted dT cap rendered a molecule composed of DNA more stable than its fYrR counterpart. By far, fully modified oligonucleotides (100% 2-O-Methyl or 2'-O-methyl A, C, and U in combination with 2'-fluoro G, termed fGmH) had the longest half-lives. These compositions demonstrated little degradation in human serum even after prolonged incubation. Together these results support the need for using fully modified aptamers for in vivo applications and should encourage those in the field to exploit newer polymerase variants capable of directly generating such polymers.

Therapies that can eliminate both local and metastatic prostate tumor lesions while sparing normal organ tissue are desperately needed. With the goal of developing an improved drug-targeting strategy, we turned to a new class of targeted anticancer therapeutics: aptamers conjugated to highly toxic chemotherapeutics. Cell selection for aptamers with prostate cancer specificity yielded the E3 aptamer, which internalizes into prostate cancer cells without targeting normal prostate cells. Chemical conjugation of E3 to the drugs monomethyl auristatin E (MMAE) and monomethyl auristatin F (MMAF) yields a potent cytotoxic agent that efficiently kills prostate cancer cells in vitro but does not affect normal prostate epithelial cells. Importantly, the E3 aptamer targets tumors in vivo and treatment with the MMAF-E3 conjugate significantly inhibits prostate cancer growth in mice, demonstrating the in vivo utility of aptamer-drug conjugates. Additionally, we report the use of antidotes to block E3 aptamer-drug conjugate cytotoxicity, providing a safety switch in the unexpected event of normal cell killing in vivo.

Pancreatic cancer is one of the most lethal malignancies. Treatment with the first-line agent, gemcitabine, is often unsuccessful because it, like other traditional chemotherapeutic agents, is non-specific, resulting in off-target effects that necessitate administration of subcurative doses. Alternatively, monomethyl auristatin E (MMAE) and monomethyl auristatin F (MMAF) are highly toxic small molecules that require ligand-targeted delivery. MMAE has already received FDA approval as a component of an anti-CD30 antibody-drug conjugate, brentuximab vedotin. However, in contrast to antibodies, aptamers have distinct advantages. They are chemicals, which allows them to be produced synthetically and facilitates the rapid development of diagnostics and therapeutics with clinical applicability. In addition, their small size allows for enhanced tissue distribution and rapid systemic clearance. Here, we assayed the toxicity of MMAE and MMAF conjugated to an anti-transferrin receptor aptamer, Waz, and an anti-epidermal growth factor receptor aptamer, E07, on the pancreatic cancer cell lines Panc-1, MIA PaCa-2, and BxPC3. In vitro, our results indicate that these aptamers are a viable option for the targeted delivery of toxic payloads to pancreatic cancer cells.

Delivery of toxins, such as the ricin A chain, Pseudomonas exotoxin, and gelonin, using antibodies has had some success in inducing specific toxicity in cancer treatments. However, these antibody-toxin conjugates, called immunotoxins, can be bulky, difficult to express, and may induce an immune response upon in vivo administration. We previously reported delivery of a recombinant variant of gelonin (rGel) by the full-length prostate-specific membrane antigen (PSMA) binding aptamer, A9, to potentially circumvent some of these problems. Here, we report a streamlined approach to generating aptamer-rGel conjugates utilizing a chemically synthesized minimized form of the A9 aptamer. Unlike the full-length A9 aptamer, this minimized variant can be chemically synthesized with a 5¢ terminal thiol. This facilitates the large scale synthesis and generation of aptamer toxin conjugates linked by a reducible disulfide linkage. Using this approach, we generated aptamertoxin conjugates and evaluated their binding specificity and toxicity. On PSMA(+) LNCaP prostate cancer cells, the A9.min-rGel conjugate demonstrated an IC 50 of *60 nM. Additionally, we performed a stability analysis of this conjugate in mouse serum where the conjugate displayed a t 1/2 of *4 h, paving the way for future in vivo experiments.

Recent advances in chemotherapy treatments are increasingly targeted therapies, with the drug conjugated to an antibody able to deliver it directly to the tumor. As high-affinity chemical ligands that are much smaller in size, aptamers are ideal for this type of drug targeting. Aptamer-highly toxic drug conjugates (ApTDCs) based on the E3 aptamer, selected on prostate cancer cells, target and inhibit prostate tumor growth in vivo. Here, we observe that E3 also broadly targets numerous other cancer types, apparently representing a universal aptamer for cancer targeting. Accordingly, ApTDCs formed by conjugation of E3 to the drugs monomethyl auristatin E (MMAE) or monomethyl auristatin F (MMAF) efficiently target and kill a range of different cancer cells. Notably, this targeting extends to both patient-derived explant (PDX) cancer cell lines and tumors, with the E3 MMAE and MMAF conjugates inhibiting PDX cell growth in vitro and with the E3 aptamer targeting PDX colorectal tumors in vivo.

Summary High-throughput sequencing can enhance the analysis of aptamer libraries generated by the Systematic Evolution of Ligands by EXponential enrichment. Robust analysis of the resulting sequenced rounds is best implemented by determining a ranked consensus of reads following the processing by multiple aptamer detection algorithms. While several such approaches have been developed to this end, their installation and implementation is problematic. We developed AptCompare, a cross-platform program that combines six of the most widely used analytical approaches for the identification of RNA aptamer motifs and uses a simple weighted ranking to order the candidate aptamers, all driven within the same GUI-enabled environment. We demonstrate AptCompare’s performance by identifying the top-ranked candidate aptamers from a previously published selection experiment in our laboratory, with follow-up bench assays demonstrating good correspondence between the sequences’ rankings and their binding affinities. Availability and implementation The source code and pre-built virtual machine images are freely available at https://bitbucket.org/shiehk/aptcompare. Supplementary information Supplementary data are available at Bioinformatics online.

High-Throughput Sequencing can enhance the analysis of aptamer libraries generated by the Systematic Evolution of Ligands by EXponential enrichment (HTS-SELEX).Robust analysis of the resulting sequenced rounds is best implemented by determining a ranked consensus of reads following the processing by multiple aptamer detection algorithms. Whilst several such approaches have been developed to this end, their installation and implementation is problematic. We developed AptCompare, a cross-platform program that combines six of the most widely used analytical approaches for the identification of RNA aptamer motifs and uses a simple weighted ranking to order the candidate aptamers, all driven within the same GUIenabled environment. We demonstrate AptCompare's performance by identifying the top-ranked candidate aptamers from a previously published selection experiment in our laboratory, with follow-up bench assays demonstrating good correspondence between the sequences' rankings and their binding affinities. Availability and Implementation:The source code and pre-built virtual machine images are freely available at https://bitbucket.org/shiehk/aptcompare.