Optical pretargeting of tumor with fluorescent MORF oligomers

Use of Morpholino Oligomers for Pretargeting

Differing from the conventional direct-targeting strategy in which a probe or payload is directly loaded onto a targeting molecule that binds to the native target, pretargeting is an improved targeting strategy. It converts the native target to an artificial target specific for a secondary targeting molecule loaded with the probe or payload (effector). The effector is small and does not accumulate in normal tissues, which accelerates the targeting process and generates high target to nontarget ratios. DNA/cDNA analogs can serve as the recognition pair, i.e., the artificial target and the secondary targeting effector. Morpholino oligomers are so far the most investigated and the most successful DNA/cDNA analog recognition pairs for pretargeting. Herein, we describe the pretargeting principles, the pretargeting strategy using Morpholino oligomers, and the preclinical success so far achieved.

Targeted amplification of delivery to cell surface receptors by dendrimer self-assembly

Nanometer-scale architectures assembled on cell surface receptors from smaller macromolecular constituents generated a large amplification of fluorescence. A targeted dendrimer was synthesized from a cystamine-core G4 PAMAM dendrimer, and contained an anti-BrE3 monoclonal antibody as the targeting group, several fluorophores and an average of 12 aldehyde moieties as complementary bio-orthogonal reactive sites for the covalent assembly. A cargo dendrimer, derived from a PAMAM G4 dendrimer, contained several fluorophores as the cargo for delivery and five hydrazine moieties as complimentary bio-orthogonal reactive sites. The system is designed to be flexible and allow for facile incorporation of a variety of targeting ligands.

Imaging mRNA Expression in Live Cells via PNA·DNA Strand Displacement-Activated Probes

Probes for monitoring mRNA expression in vivo are of great interest for the study of biological and biomedical problems, but progress has been hampered by poor signal to noise and effective means for delivering the probes into live cells. Herein we report a PNA·DNA strand displacement-activated fluorescent probe that can image the expression of iNOS (inducible nitric oxide synthase) mRNA, a marker of inflammation. The probe consists of a fluorescein labeled antisense PNA annealed to a shorter DABCYL(plus)-labeled DNA which quenches the fluorescence, but when the quencher strand is displaced by the target mRNA the fluorescence is restored. DNA was used for the quencher strand to facilitate electrostatic binding of the otherwise netural PNA strand to a cationic shell crosslinked knedel-like (cSCK) nanoparticle which can deliver the PNA·DNA duplex probe into cells with less toxicity and greater efficiency than other transfection agents. RAW 264.7 mouse macrophage cells transfected with the iNOS PNA·DNA probe via the cSCK showed a 16 to 54-fold increase in average fluorescence per cell upon iNOS stimulation. The increase was 4 to 7-fold higher than that for a non-complementary probe, thereby validating the ability of a PNA·DNA strand displacement-activated probe to image mRNA expression in vivo.

Pretargeted radioimmunotherapy for hematologic and other malignancies

Radioimmunotherapy (RIT) has emerged as one of the most promising treatment options, particularly for hematologic malignancies. However, this approach has generally been limited by a suboptimal therapeutic index (target-to-nontarget ratio) and an inability to deliver sufficient radiation doses to tumors selectively. Pretargeted RIT (PRIT) circumvents these limitations by separating the targeting vehicle from the subsequently administered therapeutic radioisotope, which binds to the tumor-localized antibody or is quickly excreted if unbound. A growing number of preclinical proof-of-principle studies demonstrate that PRIT is feasible and safe and provides improved directed radionuclide delivery to malignant cells compared with conventional RIT while sparing normal cells from nonspecific radiotoxicity. Early phase clinical studies corroborate these preclinical findings and suggest better efficacy and lesser toxicities in patients with hematologic and other malignancies. With continued research, PRIT-based treatment strategies promise to become cornerstones to improved outcomes for cancer patients despite their complexities.

Optical antisense imaging of tumor with fluorescent DNA duplexes

Antisense targeting of tumor with fluorescent conjugated DNA oligomers has the potential of improving tumor/normal tissue ratios over that achievable by nuclear antisense imaging. When administered as a linear duplex of two fluorophore-conjugated oligomers arranged in a manner that inhibits fluorescence as the duplex and designed to dissociate only in the presence of the target mRNA, the fluorescence signal should in principle be inhibited everywhere except in the target cell. Optical imaging by fluorescence quenching using linear fluorophore-conjugated oligomers has not been extensively investigated and may not have been previously considered for antisense targeting. We evaluated in cell culture and in KB-G2 tumor bearing nude mice a 25-mer phosphorothioate (PS) anti- mdr1 antisense DNA conjugated with the Cy5.5 emitter on its 3' equivalent end and hybridized as a linear duplex with a shorter 18-mer phosphodiester (PO) complementary DNA (cDNA) with the Black Hole inhibitor BHQ3 on its 5' end. In serum environments, 90% of the DNA25-Cy5.5 fluorescence was inhibited immediately following addition of the cDNA18-BHQ3 and showed only slight loss of inhibition over 24 h at 37 degrees C. As evidence of antisense specific binding, when incubated with the DNA25-Cy5.5/cDNA18-BHQ3 duplex, the fluorescence was lower in KB-31 (Pgp +/-) cells compared to KB-G2 (Pgp++) cells, but when incubated with the control cDNA18-Cy5.5/DNA25-BHQ3 duplex in which the fluorophores were reversed, the fluorescence of both cell types was low. As further evidence of specific binding, the fluorescent intensity of total RNA from KB-G2 cells incubated with the study duplex showed evidence of dissociation and hybridization with the target mRNA. Furthermore, the fluorescence microscopy images of KB-G2 cells incubated with DNA25-Cy5.5 as the singlet or study duplex show that migration in both cases is to the nucleus. The animal studies were performed in mice bearing KB-G2 tumor in one thigh and receiving iv the study or control duplexes. The tumor/normal thigh fluorescence ratio was clearly positive as early as 30 min postinjection in the study mice and reached a maximum at 5 h. By contrast, much lower fluorescence was observed in mice receiving the control duplex at the same dosage. Fluorescence microscope imaging showed that the Cy5.5 fluorescence was much higher in tumor sections from the animal that had received the study rather than control duplex. Thus combining a fluorophore-conjugated antisense DNA with an inhibitor-conjugated shorter complementary cDNA inhibited fluorescence both in cell culture and in tumored animals except in the presence of the target mRNA. This proof of concept investigation of optical antisense targeting therefore suggests that further studies including optimization of this approach are appropriate.

Comparison of several linear fluorophore- and quencher-conjugated oligomer duplexes for stability, fluorescence quenching, and kinetics in vitro and in vivo in mice

A useful property of optical imaging is the potential to modulate the detectable signal to improve target/nontarget ratios. When administered as a dimer of a fluorophore- and a quencher-conjugated duplex arranged to inhibit fluorescence but designed to dissociate only in the presence of its target, the fluorescence signal should in principle appear only in the target. This laboratory has demonstrated the feasibility of this approach by using a duplex consisting of a linear oligomer conjugated with Cy5.5 (emitter) hybridized to another linear oligomer conjugated with Iowa Black (quencher) in a pretargeting optical study. Now eight duplexes consisting of combinations of 18 mer linear phosphodiester (PO) and phosphorothioate (PS) DNAs and phosphorodiamidate morpholinos (MORFs) conjugated with Cy5.5 (emitter) and Iowa Black (quencher) were variously screened for in vitro duplex stability. The MORF/PO duplex was selected for further study based on evidence of stability in 37 degrees C serum. Simultaneously, the kinetics of quenching were investigated in vitro and in vivo in mice. Thereafter, mice were implanted in one thigh with MORF/PO Cy 5.5 microspheres and the complementary PS Iowa Black administered iv to measure the extent and kinetics of duplex formation in the target. While all duplexes were stable in buffer, only the MORF/PO duplexes and possibly all PS containing duplexes were stable in 37 degrees C serum for at least 4 h. The kinetics of quenching were found to be rapid in vitro, with a 80-90% decrease in Cy5.5 fluorescence immediately following formation of a PS/PS homoduplex, and in vivo, with a 27 to 38% decrease in target thigh/nontarget ratio within 1 h following administration of the complementary PS Iowa Black complementary DNA but not the random control DNA to mice implanted with MORF/PO Cy5.5 microspheres. This investigation has provided additional evidence that Cy5.5 may be efficiently and rapidly quenched by Iowa Black when both are conjugated to complementary oligomers and that the resulting inhibition of fluorescence is sufficiently persistent for imaging.