Antitumor activity of XR5944, a novel and potent topoisomerase poison.

Inhibitors of topoisomerases are widely used in the treatment of cancer, including inhibitors of topoisomerase I (camptothecin analogs such as irinotecan and topotecan) and topoisomerase II (etoposide and doxorubicin). The novel bis-phenazine, XR5944, is a joint inhibitor of topoisomerase I and II as shown by the stabilization of topoisomerase-dependent cleavable complexes. XR5944 demonstrated exceptional activity against human and murine tumor cells in vitro and in vivo. In a range of cell lines XR5944 (IC50 0.04-0.4 nM) was significantly more potent than TAS-103, originally proposed as a joint topoisomerase I and II inhibitor, as well as agents specific for topoisomerase I or II (topotecan, doxorubicin and etoposide). In addition, XR5944 was unaffected by atypical drug resistance and retained significant activity in cells overexpressing P-glycoprotein or multidrug resistance-associated protein. Antitumor efficacy of XR5944 was demonstrated in human carcinoma xenograft models (H69 small cell lung cancer and HT29 colon). In the HT29 model, which is relatively unresponsive to chemotherapy, XR5944 (15 mg/kg i.v., q4dx3) induced tumor regression in the majority of animals (six of eight), whereas TAS-103, dosed at its maximum tolerated dose (45 mg/kg i.v., q7dx3), only induced a delay in tumor growth compared with control animals. In the H69 model, low doses of XR5944 (5 mg/kg i.v., qdx5/week for 2 weeks or 10-15 mg/kg i.v., q4dx3), induced complete tumor regression in the majority of animals. In contrast, topotecan (20 mg/kg i.v., q4dx3) or etoposide (30 mg/kg i.v., q5dx5) only slowed the tumor growth rate. These studies show that XR5944 is a highly active novel anticancer agent that is well tolerated at efficacious doses.

In vitro and in vivo reversal of P-glycoprotein-mediated multidrug resistance by a novel potent modulator, XR9576.

The overexpression of P-glycoprotein (P-gp) on the surface of tumor cells causes multidrug resistance (MDR). This protein acts as an energy-dependent drug efflux pump reducing the intracellular concentration of structurally unrelated drugs. Modulators of P-gp function can restore the sensitivity of MDR cells to such drugs. XR9576 is a novel anthranilic acid derivative developed as a potent and specific inhibitor of P-gp, and in this study we evaluate the in vitro and in vivo modulatory activity of this compound. The in vitro activity of XR9576 was evaluated using a panel of human (H69/LX4, 2780AD) and murine (EMT6 AR1.0, MC26) MDR cell lines. XR9576 potentiated the cytotoxicity of several drugs including doxorubicin, paclitaxel, etoposide, and vincristine; complete reversal of resistance was achieved in the presence of 25-80 nM XR9576. Direct comparative studies with other modulators indicated that XR9576 was one of the most potent modulators described to date. Accumulation and efflux studies with the P-gp substrates, [3H]daunorubicin and rhodamine 123, demonstrated that XR9576 inhibited P-gp-mediated drug efflux. The inhibition of P-gp function was reversible, but the effects persisted for >22 h after removal of the modulator from the incubation medium. This is in contrast to P-gp substrates such as cyclosporin A and verapamil, which lose their activity within 60 min, suggesting that XR9576 is not transported by P-gp. Also, XR9576 was a potent inhibitor of photoaffinity labeling of P-gp by [3H]azidopine implying a direct interaction with the protein. In mice bearing the intrinsically resistant MC26 colon tumors, coadministration of XR9576 potentiated the antitumor activity of doxorubicin without a significant increase in toxicity; maximum potentiation was observed at 2.5-4.0 mg/kg dosed either i.v. or p.o. In addition, coadministration of XR9576 (6-12 mg/kg p.o.) fully restored the antitumor activity of paclitaxel, etoposide, and vincristine against two highly resistant MDR human tumor xenografts (2780AD, H69/LX4) in nude mice. Importantly all of the efficacious combination schedules appeared to be well tolerated. Furthermore, i.v. coadministration of XR9576 did not alter the plasma pharmacokinetics of paclitaxel. These results demonstrate that XR9576 is an extremely potent, selective, and effective modulator with a long duration of action. It exhibits potent i.v. and p.o. activity without apparently enhancing the plasma pharmacokinetics of paclitaxel or the toxicity of coadministered drugs. Hence, XR9576 holds great promise for the treatment of P-gp-mediated MDR cancers.

Role of lipoxygenase metabolites in platelet-activating factor- and antigen-induced bronchial hyperresponsiveness and eosinophil infiltration.

The effect of a novel leuktriene B4 receptor antagonist N-[5[[8-(1-hydroxy-2- phenyl)ethyl]dibenzofuran-2yl]5-hydroxypentanoyl]pyrrolidine (PF 10042) has been evaluated in comparison with 2-[3(1-hydroxyhexyl)phenoxymethyl]quinoline hydrochloride (PF 5901), a specific inhibitor of the 5-lipoxygenase pathway of arachidonic acid metabolism, against platelet activating factor (PAF) and allergen induced bronchial hyperresponsiveness and pulmonary eosinophil infiltration in the guinea pig. PF 10042 significantly displaced radiolabelled [3H]leukotriene B4 from binding sites on human neutrophils with an EC50 of 3 muM. PF 10042 (100 mg/kg, i.p.) significantly inhibited PAF and allergen induced bronchial hyperresponsiveness without reducing the concomitant eosinophil infiltration, whereas PF 5901 (100 mg/kg, p.o.) significantly inhibited both PAF and allergen induced bronchial hyperresponsiveness and eosinophil infiltration. We suggest from these results that PAF and allergen induced bronchial hyperresponsiveness may be secondary to the release of leukotriene B4, but this lipoxygenase metabolite does not contribute significantly to the observed eosinophil infiltration.

Effect of PF 10040 on PAF-induced airway responses in neonatally immunized rabbits.

1. PF 10040 displaced [3H]-PAF from binding sites on rabbit platelets with an IC50 = 1.07 x 10(-5) M, which was approximately three orders of magnitude below that of a standard PAF antagonist WEB 2086 (IC50 = 4.23 x 10(-9) M). 2. PF 10040 at doses of 5 and 10 mg (direct intratracheal administration) had no effect on the acute bronchoconstriction induced by PAF in neonatally immunized rabbits (airway resistance RL or dynamic compliance Cdyn). However, the PAF-induced increase in airway responsiveness to inhaled histamine was significantly inhibited (RL and Cdyn) by both doses of PF 10040. 3. PF 10040 (5 and 10 mg) significantly inhibited the total pulmonary cell infiltration and neutrophil influx induced by PAF as assessed by bronchoalveolar lavage. PAF-induced eosinophil infiltration into the airways was significantly inhibited in rabbits that received only 10 mg PF 10040. 4. We suggest from the results of the present study that PF 10040 does not exert an inhibitory effect on PAF-induced airway responses solely via antagonism of the PAF receptor located on platelets, as PF 10040 significantly inhibited PAF-induced airway hyperresponsiveness in the absence of an effect on the acute bronchospasm induced by PAF. 5. We provide further evidence that pulmonary eosinophil infiltration and the development of airway hyperresponsiveness are not causally related events as the lower dose of PF 10040 (5 mg) significantly inhibited PAF-induced airway hyperresponsiveness yet was without effect on the eosinophil influx.