Triptycenes: a novel synthetic class of bifunctional anticancer drugs that inhibit nucleoside transport, induce DNA cleavage and decrease the viability of leukemic cells in the nanomolar range in vitro.

In contrast to their inactive parent compound triptycene (code name TT0), several triptycene (TT) analogs (code names TT1 to TT13), most of them new compounds, were synthesized and shown to prevent L1210 leukemic cells from synthesizing macromolecules and growing in vitro. The most potent rigid tetracyclic quinones synthesized so far are TT2 and its C2-brominated derivative, TT13. The antitumor activity of TT2 has been compared to that of daunomycin (DAU), a clinically valuable anthracycline antibiotic which is structurally different from TT2 but also contains a quinone moiety. TT2 inhibits the proliferation (IC50: 300 nM at day 2 and 150 nM at day 4) and viability (IC50: 250 nM at day 2 and 100 nM at day 4) of L1210 cells to the same maximal degree as DAU, suggesting that the cytostatic and cytotoxic activities of TT2 are a combination of drug concentration and duration of drug exposure. Since TT2 does not increase the mitotic index of L1210 cells at 24 h like vincristine, it is unlikely to be an antimitotic drug that disrupts microtubule dynamics. Like DAU, a 1.5-3 h pretreatment with TT2 is sufficient to inhibit the rates of DNA, RNA and protein syntheses determined over 30-60 min periods of pulse-labeling in L1210 cells in vitro (IC50: 6 microM). In contrast to DAU, which is inactive, a 15 min pretreatment with TT2 has the advantage of also inhibiting the cellular transport of nucleosides occuring over a 30 s period in vitro (IC50: 6 microM), suggesting that TT2 prevents the incorporation of [3H]thymidine into DNA because it rapidly blocks the uptake of [3H]thymidine by the tumor cells. After 24 h, TT2 induces as much DNA cleavage as camptothecin and DAU, two anti-cancer drugs producing DNA strand breaks and known to respectively inhibit DNA topoisomerase I and II activities. Interestingly, the abilities of TT2 to block nucleoside transport, inhibit DNA synthesis and induce DNA fragmentation are irreversible upon drug removal, suggesting that this compound may rapidly interact with various molecular targets in cell membranes and nuclei to disrupt the functions of nucleoside transporters and nucleic acids, and trigger long-lasting antitumor effects which persist after cessation of drug treatment. Because inhibition of nucleoside transport is highly unusual among DNA-damaging drugs, the use of bifunctional TTs with antileukemic activity in the nM range in vitro might provide a considerable advantage in polychemotherapy to potentiate the action of antimetabolites and sensitize multidrug-resistant tumor cells.