Molecular imaging of Bcr-Abl phosphokinase in a xenograft model

The purpose of this study was to determine whether the Bcr-Abl tyrosine kinase can be assessed by gamma-imaging using an 111In-labeled anti-phosphotyrosine (APT) antibody, and if the response to treatment with imatinib could be detected using this imaging technique. APT antibody was labeled with 111In using ethylenedicysteine (EC) as a chelator. To determine if 111In-EC-APT could assess a nonreceptor tyrosine kinase, xenografts of the human chronic myelogenous leukemia cell line K562 were used. gamma-Scintigraphy of the tumor-bearing mice, before and after imatinib treatment, was obtained 1, 24, and 48 h after they were given 111In-EC-APT (100 microCi/mouse i.v.). 111In-EC-APT is preferentially taken up by Bcr-Abl-bearing tumor cells when compared with 111In-EC-BSA or 111In-EC-IgG1 controls and comparable with the level of uptake of 111In-EC-Bcr-Abl. Imatinib treatment resulted in decreased expression of phospho-Bcr-Abl by Western blot analysis, which correlated with early (4 days after starting imatinib) kinase down-regulation as assessed by imaging using 111In-EC-APT. The optimal time to imaging was 24 and 48 h after injection of 111In-EC-APT. Although tumor regression was insignificant on day 4 after starting imatinib treatment, it was marked by day 14. 111In-EC-APT can assess intracellular phosphokinase activity, and down-regulation of phosphokinase activity predates tumor regression. This technique may therefore be useful in the clinic to detect the presence of phosphokinase activity and for early prediction of response.

Cytotoxic activity of a new lipid formulation of doxorubicin in cell lines and primary tumor cells

BACKGROUND

Liposomal formulations of the anthracyclines are being developed to circumvent toxicity and prolong effect. The current study investigates the in vitro activity of a novel doxorubicin micelle formulation, containing a vehicle designed to release pharmacologically active subcomponents.

MATERIALS AND METHODS

The cytotoxicity of doxorubicin formulated in a vehicle containing C4 (N-docosahexaenoyl-O-phospho-2-aminoethanol) and C11 (N-all trans-retinoyl-O-phospho-L-tyrosine) was measured in a panel of human tumor cell lines, 19 primary cultures of human tumor cells and 5 lymphocyte preparations.

RESULTS AND CONCLUSION

At the tested ratio between doxorubicin and C4/C11 (1:50), C4/C11 contributed significantly to the in vitro toxicity. However, the molar EC50-values were lower for doxorubicin than for C4/C11. Synergistic interactions between doxorubicin and C4/C11 were evident in a majority of the cell types studied. C4/C11 increased the cellular load of the fluorescent Pgp substrate calcein. To further investigate the possible benefits of the new formulation, in vivo studies are ongoing.

Understanding the cellular uptake of phosphopeptides

Phosphopeptide-cellular uptake has been studied with a unique combination of tools designed to quantitate this phenomena and to understand properties that contribute to transmembrane penetration. High-affinity src-homology domain (SH2) hexapeptides for the phosphatidyl inositol 3-kinase system were used to judge cell penetration using red blood cells--a model system for the study of transmembrane cellular uptake. Hexapeptides without phosphate groups and devoid of charged residues poorly entered cells. N-terminal modification with bulky hydrophobic groups enhanced partitioning into octanol, an index of hydrophobicity, and allowed certain non-phosphorylated peptides to pass into red cells. However, tyrosine phosphorylation of hexapeptides markedly decreased octanol-water partitioning and completely eliminated cellular uptake. Inclusion of ion-pairing agents that masked the phosphate hydrophilic character enabled partitioning of phosphopeptides into octanol and achieved cellular uptake. This effect was demonstrated using fluorescent derivatives of phosphopeptides and CV1 cells in culture. The results validate the concept of facilitating cell entry by charge masking and open the way to future refinements of this principle. Various penetration techniques are compared and discussed in the context of maximizing cellular viability.

Potent inhibition of protein-tyrosine phosphatase by phosphotyrosine-mimic containing cyclic peptides

In an effort to derive potent and bioavailable protein-tyrosine phosphatase inhibitors, we have previously reported hexameric peptides based on the epidermal growth factor receptor sequence EGFR988-993 (Asp-Ala-Asp-Glu-Xxx-Leu, where Xxx = Tyr), in which the tyrosyl residue has been replaced by the non-hydrolyzable phosphotyrosyl mimics phosphonomethylphenylalanine (Pmp), difluorophosphonomethylphenylalanine (F2Pmp) and O-malonyltyrosine (OMT). Inhibitory potencies (IC50 values) of these peptides against the tyrosine phosphatase PTP IB were 200, 0.2 and 10 microM, respectively. Since cellular penetration of peptides containing highly charged phosphonate residues is compromised, and good bioreversible protection strategies for the F2Pmp residue have not yet been reported, the OMT residue is of particular interest in that it affords potential new prodrug approaches. In the current study we have prepared cyclized versions of the OMT-containing EGFR988-993 peptide in order to increase its proteolytic stability and restrain conformational flexibility. Three different cyclic analogues were synthesized. Two of these were cyclized through the peptide backbone ('head to tail') using in one case a single glycine spacer (heptamer peptide) and in the second instance, two glycines (octamer peptide). In a PTPI-based assay the cyclic heptamer experienced a two-fold loss of potency (Ki = 25.2 +/- 3.9 microM) relative to the linear hexamer parent (Ki = 13 +/- 0.9 microM), while the cyclic octamer demonstrated a live-fold increase in potency (Ki = 2.60 +/- 0.11 microM). The third peptide was cyclized by means of a sulfide bridge between the side chain of a C-terminally added cysteine residue and the beta-carbon of a N-terminal acetyl residue. Although the overall size of this ring was identical to that exhibited by the preceding backbone-cyclized octamer, it displayed a three-fold enhancement in potency (Ki = 0.73 +/- 0.03 microM). The structural basis for the observed results are discussed. Conformation restrictions induced by cyclization could aid in defining geometries for peptidomimetic design. Finally, it can be speculated that cyclization of other liner PTP-inhibitory peptides, such as the F2Pmp-containing hexamer, may also increase their potency.