Time-resolved fluorescence intensity decay data from anthracycline anticancer drugs present in model membranes were obtained using a gigahertz frequency-domain fluorometer [Lakowicz et al. (1986) Rev. Sci. Instrum. 57, 2499-2506]. Exciting light of 290 nm, modulated at multiple frequencies from 8 MHz to 400 MHz, was used to study the interactions of Adriamycin, daunomycin and related antibiotics with small unilamellar vesicles composed of dimyristoylphosphatidylcholine (DMPC) at 28°C. Fluorescence decay data for drug molecules free in solution as well as bound to membranes were best fit by exponentials requiring two terms rather than by single exponential decays. For example, one-component analysis of the decay data for Adriamycin free in phosphate buffered saline (PBS) solution resulted in a reduced x{sup}2 value of 140 (r = 0.88 ns), while a two-component fit resulted in a substantially smaller reduced x{sup}2 value of 2.6 (r{sub}1 = 1.13 ns, α{sub}1 = 0.60, r{sub}2 = 0.30 ns). Upon association with membranes, each of the anthracyclines studied displayed a larger r{sub}1 value while the r{sub}2 value remained the same or increased (for example, DMPC-bound Adriamycin showed r{sub}1 = 1.68 ns, α{sub}1 = 0.64, r{sub}2 = 0.33 ns). Analyses of the fluorescence emission decays of anthracyclines were also made assuming each decay is composed of a single Lorentzian distribution of lifetimes. Data taken on Adriamycin in PBS, when fit using one continuous component, displayed r, α, w, and reduced x{sup}2 values of 0.68 ns, 1, 0.60 ns, and 9.1, respectively. The distribution became quite broad upon drug association with membrane (DMPC-bound Adriamycin: r = 0.75 ns, α = 1, w = 2.24 ns, x{sup}2 = 13). For each anthracycline studied, continuous component fits showed significant broadening in the distributions upon drug association with membrane. Relatively large shifts in lifetime values were observed for the carminomycin and 4-demethoxy-daunomycin analogues upon binding model lipid membranes, making these agents good candidates to employ in future studies on anthracycline interactions with more environmentally-complex biological membranes.
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