Gadolinium texaphyrin (Gd-Tex)-malonato-platinum conjugates: synthesis and comparison with carboplatin in normal and Pt-resistant cell lines

Abstract

The synthesis of a new PEG-solubilized gadolinium texaphyrin (Gd-Tex) conjugate containing a malonate-Pt(NH(3))(2) moiety is described. The effect of the tumor localizing Gd-Tex macrocycle on platinum activity was evaluated in cell culture. The malonate moiety, analogous to that present in carboplatin, is expected to release an aquated Pt(NH(3))(2) species under physiological conditions. The half-life in phosphate-buffered saline was found to be ca. 3 days at room temperature, and the hydrolytic product released from the conjugate was collected and confirmed as Pt-based by flameless atomic absorption spectrophotometry. Anti-proliferative activity was tested using A549 human lung cancer and A2780 human ovarian cancer cell lines. In both cell lines, the activity of the Gd-Tex conjugate was found to be similar to that of carboplatin. Efficacy against a Pt-resistant ovarian cell line greater than that displayed by carboplatin was also observed.

Figure 1

Structures of cisplatin, carboplatin, oxaliplatin, motexafin gadolinium, MGd 1, Gd-Tex-amine-Pt conjugate 2, Gd-Tex-aminosuccinate–Pt conjugate 3.

Figure 2

Malonate-type Pt conjugates: aminomalonate 4, PEG-amidomalonate 5, and folate-PEG 6. AA – amino acid.

Figure 3

(a) HPLC hydrolysis study of Gd-Tex-Pt conjugate 7 in methanol (grey line) and phosphate buffered saline (PBS) (black line). Samples in PBS display a t_1/2 of ca. 3 days. (b) Pt concentration in eluates collected from a Sep Pak column pre-loaded with an aliquot of conjugate 7 in PBS incubated over 4 days. Each fraction represents 1 mL of eluent.

Figure 4

Effect of conjugate 7 on cell proliferation. A549 lung cancer cells were incubated with inhibitors for 3 days at 37 °C in 5% CO₂ and relative proliferation measured by tetrazolium salt reduction. The anti-proliferative activity of MGd, carboplatin, or the combined treatment is shown for comparison. Error bars represent standard deviation.

Figure 5

MTT Cell proliferation assay with A2780 (a) and cisplatin resistant 2780CP (b) ovarian cancer cell lines in the presence of variable concentrations of indicated drug. The drug was added to cells and incubated for 5 days at 37 °C in 5% CO₂. Error bars represent standard deviation.

Scheme 1

Synthesis of Gd-Tex-propylmalonate Pt conjugate 7.