Seite - 61 - in Cancer Nanotheranostics - What Have We Learnd So Far?

PERSPECTIVEARTICLE published:14October2014 doi: 10.3389/fchem.2014.00087 Therapeuticantisenseoligonucleotidesagainst cancer: hurdling to theclinic PedroM.D.Moreno1*andAnaP.Pêgo1,2,3 1 Instituto de Engenharia Biomédica, Nanobiomaterials for Targeted Therapies Group, Porto, Portugal 2 Faculdade de Engenharia, Universidade do Porto, Porto, Portugal 3 InstitutodeCiênciasBiomédicas AbelSalazar,Universidade doPorto, Porto, Portugal Editedby: João Conde, Massachusetts Institute of Technology, USA Reviewedby: Juewen Liu, Universityof Waterloo, Canada Ramon Eritja, InstitutdeQuímica Avançada deCatalunya -Consejo Superior de Investigaciones Científicas, Spain *Correspondence: Pedro M. D.Moreno, Instituto de Engenharia Biomédica, Nanobiomaterials for Targeted Therapies Group, Rua do Campo Alegre, 823, Porto, Portugal e-mail: pedro.moreno@ineb.up.pt Under clinical development since the early 90’s and with two successfully approved drugs (Fomivirsen and Mipomersen), oligonucleotide-based therapeutics has not yet delivered a clinical drug to themarket in the cancer field.Whilst many pre-clinical data has been generated, a lack of understanding still exists on how to efficiently tackle all the different challenges presented for cancer targeting in a clinical setting. Namely, effective drug vectorization, careful choice of target gene or synergistic multi-gene targeting are surely decisive, while caution must be exerted to avoid potential toxic, often misleading off-target-effects. Here a brief overview will be given on the nucleic acid chemistry advances that established oligonucleotide technologies as a promising therapeutic alternative and ongoing cancer related clinical trials. Special attentionwill be given toward aperspective on thehurdles encountered specifically in the cancer field by this class of therapeutic oligonucleotides and a viewonpossible avenues for success is presented,withparticular focuson thecontribution fromnanotechnology to thefield. Keywords:antisense,oligonucleotides, cancer, therapeutics,nanomedicine OPENINGTHETHERAPEUTICLANDSCAPEBYEVOLUTION OFNUCLEICACIDSCHEMISTRY Oligonucleotides have been under investigation for over 30 years, whilst achieving only two approved drugs. Those were, Fomivirsen, approved by the FDA in 1998 for the treatment of cytomegalovirus retinitis in patients with AIDS, but discontin- ued for low demand, andMipomersen, FDA approved in 2013, targeting ApoB100 for the treatment of homozygous familial hypercholesterolaemia(HoFH),a raregeneticdisorder that leads to excessive levels of low-density lipoprotein (LDL) cholesterol. These are both single-stranded antisense oligonucleotide drugs (most commonly known asAONs) that togetherwith siRNA (a double-strandedoligonucleotide)makeup, atpresent, the thera- peuticantisenseoligonucleotidefield. In thispapermoreempha- siswill beputonAONsdue to their longer time indevelopment andhistoryofclinical trials. Progress inthisfieldhasbeenproceedingatasteadybutsome- what slowpace,drivenmostlyby thespeedatwhich thedifferent intra and extracellular obstacles encountered by the oligonu- cleotidedrugsarebeingtackled.Themostimportanthurdleshave been (i) thepoor stability against extra- and intracellular degra- dation(mostlybyactionofnucleases),(ii) inefficientintracellular delivery to target cells or tissues, (iii) inadequate affinity toward the intendedtarget sequenceand(iv)potentialoff-target/toxicity effects. Finally formost applications (v) immunostimulationhas alsobeenamatterofconcern. The pursuit of clinically relevant antisense drugs has led the fieldtodevelopdifferenttypesofchemicalmodificationstonative DNA or RNA in an attempt to overcome the aforementioned limitations. Most widely used modifications can be divided in two simple categories: (a)backbone structure and (b) sugar ring modifications(Table1). Themain goals of these chemicalmodifications have been to achieve increased resistance to degradation by exo- and endo- nucleases; increase affinity, and in some cases selectivity, toward targetRNA/DNAsequencesand tomodulate the immunostimu- lationpropertiesof theoligonucleotides. In the case of phosphorothioate (PS) modification (one of the first andwidely usedmodifications introduced in therapeu- tic antisenseoligonucleotides) (Eckstein, 1967), it has also led to betterpharmacokineticsandextendedcirculationtimesforAONs systemicallyappliedina“naked”form(i.e.,unprotectedbydeliv- ery agents).This effect hasbeenattributed tounspecificbinding toserumproteinssuchasalbumin(Srinivasanetal.,1995;Crooke etal., 1996;Watanabeetal., 2006). Mechanistically,AONsworkviabindingtoaspecificRNAtar- get sequence resulting in the block of RNA function. This can beachieved throughsterichindrance (non-degradativepathway) and concomitant RNA translation block, or target degradation. The latteroccursby theactionof anendogenousenzyme,RNase H, or alternatively, by a catalytic cleavage activity embedded into the oligonucleotide itself (e.g., ribozymes andDNAzymes) (BennettandSwayze,2010).Notably, thenon-degradativemech- anism, through steric hindrance, has recently been exploited, with great success, formodulation of pre-mRNAsplice patterns by affecting the binding of trans-splicing regulatory factors to thepre-mRNA(HammondandWood, 2011; Bestas et al., 2014; Distereretal., 2014). www.frontiersin.org October2014 |Volume2 |Article87 | 61