GTMB 7 - Gene Therapy & Molecular Biology

Kairemo et al: Oligonucleotide radiotherapydevelopment (Korhonen, 1992). It is abundantly expressedin vascular endothelia during development, and in somemegakaryoblastic and erythroleukemia cell lines; as wellas tieRNA accumulates in the epithelium of local vesselsduring ovulation and wound healing (Korhonen, 1992).Tie receptor has an important role in the angiogenesisassociated with melanoma metastasis (Kaipainen, 1994).Radioantibodies against tie receptor have been used intargeting studies in vivo with success (Kairemo et al,1996). As the location of tie receptor is at the outer cellmembrane, receptor is easily reachable and effects ofradiation and receptor blocking should occur immediately,which may be beneficiary for the radioantibody treatment.For further development of these receptors the crucialpoint is to find inducers for normally low levels. Ligandsfor endothelial cell receptors tyrosine kinases, Tie-1 andTie-2 are not known. Ligands with agonistic andantagonistic activities for Tie-2 have now been identified:angiopoetin 1 is an activating ligand for Tie 2 andregulates blood vessel maturation (Suri, 1996), whileangiopoetin 2 serves as antagonist (Maisonpierre, 1997).The ETS family proteins are transcription factors thatbind to the regulatory control region of certain genes viaETS binding motif, which has been found in numerousgenes including proteases and receptor tyrosine kinases(Wasylyk, 1993). ETS regulates the expression ofproteases and migration of endothelial cells, and in fact,the induction of ETS-1 mRNA is a mutual phenomenon inendothelial cells stimulated with angiogenic growthfactors (Iwasaka, 1996). It has also been shown that ETS -1 antisense oligo markedly reduced the DNA- ETScomplex diminishing the responsiveness to the stimulus ofangiogenic factor (Iwasaka, 1996). Induction of expressionof ETS gene is faster and more prominent than proteinexpression providing better although transient target fortherapy.The specificity resides in the sequence of oligo,which interacts with its complementary mRNA, but onlyminimally with noncomplementary structures. Theantisense oligo, through the formation of a mRNA-DNAduplex, specifically prevents the translation of that mRNAinto protein (Figure 1). For oligos to be effective antisenseagents, they first must enter the cells and achieveappropriate concentration in the correct intracellularcompartment. Cellular nucleases are highly potent indigesting phosphodiester oligos. Thus several nucleaseresistant oligos have been developed. Phosphorothioateoligo has a non-bridging oxygen atom replacing a sulphuratom. Peptide nucleic acid (PNA) is an oligomer in whichthe charged phosphate-ribose backbone has beeneliminated and replaced with an uncharged backbone(Egholm 1992) and PNAs have been reported to resistnuclease and protease degradation (Egholm 1993).Oligos bind to serum albumin and other proteins withlow affinity and distribute to all peripheral tissues with thekidneys and liver accumulating most of the drug. They arecleared by slow metabolism with an elimination half-lifeup to 50 hrs. The biokinetics of GEM 91 phosphorothioateoligodeoxynucleotide has been evaluated in six AIDSpatients, where the plasma mean residence time variedfrom 24.7 to 49.6 hrs, the mean being 41.7 ± 3.6 hrs(Zhang, 1995a).Figure 1. Schematic presentation of radionanotargeting16