Structural features facilitating tumor cell targeting and internalization by bleomycin and its disaccharide.

We have shown previously that the bleomycin (BLM) carbohydrate moiety can recapitulate the tumor cell targeting effects of the entire BLM molecule, that BLM itself is modular in nature consisting of a DNA-cleaving aglycone which is delivered selectively to the interior of tumor cells by its carbohydrate moiety, and that there are disaccharides structurally related to the BLM disaccharide which are more efficient than the natural disaccharide at tumor cell targeting/uptake. Because BLM sugars can deliver molecular cargoes selectively to tumor cells, and thus potentially form the basis for a novel antitumor strategy, it seemed important to consider additional structural features capable of affecting the efficiency of tumor cell recognition and delivery. These included the effects of sugar polyvalency and net charge (at physiological pH) on tumor cell recognition, internalization, and trafficking. Since these parameters have been shown to affect cell surface recognition, internalization, and distribution in other contexts, this study has sought to define the effects of these structural features on tumor cell recognition by bleomycin and its disaccharide. We demonstrate that both can have a significant effect on tumor cell binding/internalization, and present data which suggests that the metal ions normally bound by bleomycin following clinical administration may significantly contribute to the efficiency of tumor cell uptake, in addition to their characterized function in DNA cleavage. A BLM disaccharide-Cy5** conjugate incorporating the positively charged dipeptide d-Lys-d-Lys was found to associate with both the mitochondria and the nuclear envelope of DU145 cells, suggesting possible cellular targets for BLM disaccharide-cytotoxin conjugates.

The disaccharide moiety of bleomycin facilitates uptake by cancer cells.

The disaccharide moiety is responsible for the tumor cell targeting properties of bleomycin (BLM). While the aglycon (deglycobleomycin) mediates DNA cleavage in much the same fashion as bleomycin, it exhibits diminished cytotoxicity in comparison to BLM. These findings suggested that BLM might be modular in nature, composed of tumor-seeking and tumoricidal domains. To explore this possibility, BLM analogues were prepared in which the disaccharide moiety was attached to deglycobleomycin at novel positions, namely, via the threonine moiety or C-terminal substituent. The analogues were compared with BLM and deglycoBLM for DNA cleavage, cancer cell uptake, and cytotoxic activity. BLM is more potent than deglycoBLM in supercoiled plasmid DNA relaxation, while the analogue having the disaccharide on threonine was less active than deglycoBLM and the analogue containing the C-terminal disaccharide was slightly more potent. While having unexceptional DNA cleavage potencies, both glycosylated analogues were more cytotoxic to cultured DU145 prostate cancer cells than deglycoBLM. Dye-labeled conjugates of the cytotoxic BLM aglycons were used in imaging experiments to determine the extent of cell uptake. The rank order of internalization efficiencies was the same as their order of cytotoxicities toward DU145 cells. These findings establish a role for the BLM disaccharide in tumor targeting/uptake and suggest that the disaccharide moiety may be capable of delivering other cytotoxins to cancer cells. While the mechanism responsible for uptake of the BLM disaccharide selectively by tumor cells has not yet been established, data are presented which suggest that the metabolic shift to glycolysis in cancer cells may provide the vehicle for selective internalization.

Selective tumor cell targeting by the disaccharide moiety of bleomycin.

In a recent study, the well-documented tumor targeting properties of the antitumor agent bleomycin (BLM) were studied in cell culture using microbubbles that had been derivatized with multiple copies of BLM. It was shown that BLM selectively targeted MCF-7 human breast carcinoma cells but not the "normal" breast cell line MCF-10A. Furthermore, it was found that the BLM analogue deglycobleomycin, which lacks the disaccharide moiety of BLM, did not target either cell line, indicating that the BLM disaccharide moiety is necessary for tumor selectivity. Not resolved in the earlier study were the issues of whether the BLM disaccharide moiety alone is sufficient for tumor cell targeting and the possible cellular uptake of the disaccharide. In the present study, we conjugated BLM, deglycoBLM, and BLM disaccharide to the cyanine dye Cy5**. It was found that the BLM and BLM disaccharide conjugates, but not the deglycoBLM conjugate, bound selectively to MCF-7 cells and were internalized. The same was also true for the prostate cancer cell line DU-145 (but not for normal PZ-HPV-7 prostate cells) and for the pancreatic cancer cell line BxPC-3 (but not for normal SVR A221a pancreas cells). The targeting efficiency of the disaccharide was only slightly less than that of BLM in MCF-7 and DU-145 cells and comparable to that of BLM in BxPC-3 cells. These results establish that the BLM disaccharide is both necessary and sufficient for tumor cell targeting, a finding with obvious implications for the design of novel tumor imaging and therapeutic agents.

Dynamics of bleomycin interaction with a strongly bound hairpin DNA substrate, and implications for cleavage of the bound DNA.

Recent studies involving DNAs bound strongly by bleomycins have documented that such DNAs are degraded by the antitumor antibiotic with characteristics different from those observed when studying the cleavage of randomly chosen DNAs in the presence of excess Fe·BLM. In the present study, surface plasmon resonance has been used to characterize the dynamics of BLM B(2) binding to a strongly bound hairpin DNA, to define the effects of Fe(3+), salt, and temperature on BLM-DNA interaction. One strong primary DNA binding site, and at least one much weaker site, were documented. In contrast, more than one strong cleavage site was found, an observation also made for two other hairpin DNAs. Evidence is presented for BLM equilibration between the stronger and weaker binding sites in a way that renders BLM unavailable to other, less strongly bound DNAs. Thus, enhanced binding to a given site does not necessarily result in increased DNA degradation at that site; i.e., for strongly bound DNAs, the facility of DNA cleavage must involve other parameters in addition to the intrinsic rate of C-4' H atom abstraction from DNA sugars.