Seite - 62 - in Cancer Nanotheranostics - What Have We Learnd So Far?

MorenoandPêgo AONcancer therapeutics Table1 |Commonnucleicacidsmodificationsdividedbycategory. BACKBONESTRUCTURE SUGARRING O O O S OP H H H H B X Phosphorothioate (PS) O O O O OP H H H H B O CH3 2′O-Methyl (2′O-Me) O XNH HH HH PO O O- B N′3Phosphoramidate (NP) O O O O OP H H H H B O O-CH3 2′O-Metoxyethyl (MOE) NH O O N B PeptideNucleicAcid (PNA) O O O O OP H H H B O LockedNucleicAcid (LNA) O N O O OP B O Morpholino (PMO) O OHO HH HH PO O O- B UnlockedNucleicAcid (UNA) O O FH HH PO O O- B 2′F-ArabinoNucleicAcid (2′F-ANA) Themechanism of action of AONs has to be carefully con- sidered when deciding for the type of nucleotide modifica- tions and design of the oligonucleotide (here including number and position of modified nucleotides). Thus, in contrast to PS modifications that maintain the anionic character of oligonu- cleotides,PNAsandPMOscompletely substitute thephosphodi- ester linkagebyeitherapolyamidebackbone(Nielsenetal.,1991) or aphosphorodiamidate group(SummertonandWeller, 1997), respectively, hence being uncharged nucleotide analogs. On the otherhandsugarringmodificationscan influence thenucleoside conformationpromoting thepreferential adoptionof anA-form (dsRNAtype) orB-form(dsDNAtype)helixwhen in adouble- stranded structure. In the case of 2′O-Me RNA (Kawai et al., 1992;Nishizaki et al., 1997),MOE(Manoharan, 1999) andLNA (Koshkinet al., 1998;Obikaet al., 1998), allpromote theA-form while 2′F-ANA (Berger et al., 1998) the B-form.Contrasting all the aforementioned,UNA,with its unlocked ring configuration, does not impart any conformation restrictions (Pasternak and Wengel,2011). All of the abovementioned nucleotide modifications have thus been usedwith few restrictionswhen designing steric hin- dranceAONs, since their incorporationmainly focusonachiev- ing enhanced binding affinity and selectivity toward a target sequence. In contrast, the design of AONs for target degrada- tion through the action of RNAse H has to obey the enzyme’s structuralpreferences forcleavingDNA/RNAduplexes(Minshull and Hunt, 1986; Nakamura et al., 1991). Hence, all modifica- tions too divergent from natural DNA nucleotides need to be carefully considered to not hinder the enzyme action. This can be accomplished by the design of “gapmer” AONs containing the modified nucleotides on the 5′ and 3′ terminus flanking a central unmodifiedDNAnucleotides stretch (Monia et al., 1993; Stantonet al., 2012). Specifically, in thecaseof the twoapproved drugs,Fomivirsen is aPSmodifiedDNAoligonucleotideapplied by intraocular injection,whereasMipomersen is a secondgener- ationAONgapmerwithMOEmodifications at the ends andPS throughout,appliedasan intravascular injection. BRIEFOVERVIEWONANTISENSEOLIGONUCLEOTIDES CLINICALTRIALSRELATEDTOCANCER An increasing number of clinical trials withAONs are ongoing, which shows that thefield is rapidly forwarding. InTable2 a list of recentlycompletedandon-goingclinical trials ispresented. Other studies have unfortunately failed, in different phases, to reach their expected endpoints or to show significant benefit, leading to a stop in the correspondingAONdevelopment. Some aspects of antisense technology have contributed to this and are nextdiscussed. CHALLENGESFORANTISENSETECHNOLOGY—1. UNSPECIFICMODESOFACTION Along their development path, oligonucleotides have unraveled muchof theirpotentialbutalsomanyof their limitations. As discussed above, introduction of PSmodifications led to thefirstevidencesthatantisensedrugscouldbecomeareality ina clinical setting, essentiallyby increasingresistance todegradation and extending circulation times after systemic administration (mostly due to unspecific serum-protein binding). These prop- erties improvedtheoligonucleotidetherapeuticpotential,despite somedecreasedaffinity for the target sequence(whencomparing toregularDNAoligonucleotides)(Kibler-Herzogetal.,1991).On Frontiers inChemistry | ChemicalEngineering October2014 |Volume2 |Article87 | 62