Designing the polyamine pharmacophore: influence of N-substituents on the transport behavior of polyamine conjugates

N-Ethylated N-arylmethyl polyamine conjugates were synthesized and evaluated for their ability to target the polyamine transporter (PAT). To understand the effect of N-ethylation upon PAT selectivity, ethyl groups were appended onto a PAT-selective N (1)-anthracenenylmethyl homospermidine derivative, 1b. Bioevaluation in L1210 murine leukemia cells and in two Chinese hamster ovary cell lines (PAT-active CHO and PAT-deficient CHO-MG) revealed a dramatic decrease in PAT targeting ability upon N (1) or N (5) ethylation of the pharmacophore 1b. Experiments using the amine oxidase inhibitor, aminoguanidine (AG, 2 mM), revealed that the N (9)-ethyl and N (9)-methyl analogues were able to retain their PAT selectivity and cytotoxicity properties in the presence or absence of AG. In contrast, the lead compound 1b (containing a terminal NH 2 group) revealed a dramatic reduction in both its PAT-targeting ability and cytotoxicity in the absence of AG. An improved balance between these three properties of PAT-targeting, cytotoxicity and metabolic stability can be attained via N-methylation at the N (9)-position.     

Targeting the polyamine transport system with benzazepine- and azepine-polyamine conjugates

The polyamine transport system (PTS) whose activity is up-regulated in cancer cells is an attractive target for drug design. Two heterocyclic (azepine and benzazepine) systems were conjugated to various polyamine moieties through an amidine bound to afford 18 compounds which were evaluated for their affinity for the PTS and their ability to use the PTS for cell delivery. Structure-activity relationship studies and lead optimization afforded two attractive PTS targeting compounds. The azepine-spermidine conjugate 14 is a very selective substrate of the PTS that may serve as a vector for radioelements used for diagnoses or therapeutics in nuclear medicine. The nitrobenzazepine-spermine conjugate 28 is a very powerful PTS inhibitor with very low intrinsic cytotoxicity, able to prevent the growth of polyamine depleted cells in presence of exogenous polyamines.     

Effect of polyamine homologation on the transport and biological properties of heterocyclic amidines

Five sets of heterocyclic derivatives of various sizes and complexities coupled by an amidine function to putrescine, spermidine, or spermine were prepared. They were essentially tested to determine the influence of the polyamine chain on their cellular transport. To comment on affinity and on selective transport via the polyamine transport system (PTS), K(i) values for polyamine uptake were determined in L1210 cells, and the cytotoxicity and accumulation of the conjugates were determined in CHO and polyamine transport-deficient mutant CHO-MG cells, as well as in L1210 and alpha-difluoromethylornithine- (DFMO-) treated L1210 cells. Unlike spermine, putrescine and spermidine were clearly identified as selective motifs that enable cellular entry via the PTS. However, this property was clearly limited by the size of substituents: these polyamines were able to ferry a dihydroquinoline system via the PTS but did not impart any selectivity to bulkier substituents.     

Molecular requirements for targeting the polyamine transport system. Synthesis and biological evaluation of polyamine-anthracene conjugates

A series of nine N(1)-(9-anthracenylmethyl)tetraamines (e.g., Ant-4,4,4-tetraamine) were synthesized and evaluated for cytotoxicity in L1210, alpha-difluoromethylornithine (DFMO)-treated L1210, Chinese hamster ovary (CHO), and CHO-MG cell lines. Surprisingly, the 3,3,4- and 3,4,3-tetraamine motifs had the same or decreased cytotoxicity in DFMO-treated L1210 cells, whereas the rest of the tetraamine systems were usually more cytotoxic and gave lower IC(50) values in this treated cell line. The most sensitive derivatives to DFMO treatment were the Ant-4,4,3- and Ant-4,4,4-tetraamine analogues, which were 7 and 5 times more cytotoxic in DFMO-treated L1210 cells, respectively. K(i) values for each of the anthracenylmethyl(Ant)-polyamine conjugates were determined in L1210 cells and revealed that these systems are high-affinity ligands for the polyamine transporter (PAT). Mixed results were observed in the CHO and CHO-MG assays. The 4,4,4- and 5,4,4-tetraamine motifs were 3 times more toxic to CHO cells with active polyamine transporters. For example, the Ant-4,4,4-tetraamine conjugate displayed IC(50) values of 11 microM in CHO cells and 33 microM in CHO-MG cells, a PAT-deficient cell line. This suggested that these derivatives used the PAT in part to access cells. However, most of the other tetraamine derivatives had similar potencies in both the CHO and CHO-MG cell lines. In terms of vector design, higher affinity for the PAT (lower K(i) values) did not translate into higher potency for the tetraamine conjugate. In contrast, the related triamine systems, which had micromolar K(i) values in L1210 cells, were more efficacious and selective. In one case, the 4,4-triamine motif imparted 150-fold higher potency in CHO cells than the CHO-MG mutant. A deconvolution microscopy study in A375 melanoma cells revealed a rapid internalization of the Ant-4,4-triamine as fluorescent vesicles, whereas the Ant-4,4,4-tetraamine remained mostly at the cell surface. These findings help define the key characteristics required for selective delivery of polyamine-drug conjugates into cell types with active polyamine transporters.     

Influence of the spacer on the inhibitory effect of different polycarbophil-protease inhibitor conjugates.

Within the present study various polycarbophil (PCP)-serine protease inhibitor conjugates were synthesized and the influence of different spacers on their inhibitory efficacy was evaluated in vitro. Results demonstrated that 4.2+/-0.15 units (n=3; +/-SD) of alpha-chymotrypsin were inhibited by 50% utilizing 0.86% (w/v) of a PCP-tetramethylenediamine (TMDA)-chymostatin 20:1 conjugate. In contrast, only 0.6+/-0.05 units (n=3; +/-SD) of alpha-chymotrypsin were inhibited by a corresponding PCP-poly(ethylene glycol) (PEG)-chymostatin conjugate. Inhibitory effects of PCP-TMDA-antipain and -elastatinal conjugates towards trypsin and elastase, respectively, were also significantly higher (P<0.05) than those of corresponding PCP-PEG-inhibitor conjugates. Hence, the great impact of the molecular size as well as the structure of the spacer on resulting polymer-inhibitor conjugates could be demonstrated. The small and rigid C4-spacer TMDA (molecular weight (MW) 161.1) was thereby shown to be highly advantageous over a long, hydrophilic and flexible PEG-diamine spacer (MW 3400). Results obtained should provide helpful basic knowledge for the development of mucoadhesive polymer-inhibitor conjugates used as auxiliary agents for the oral administration of peptide drugs.     

Triterpene constituents from the stem bark of Pinus luchuensis and their DNA topoisomerase II inhibitory effect

Nine lanostane and serratane-type triterpenes including two unknown compounds were isolated from the stem bark of Pinus luchuensis. These new compounds were characterized as 3-oxolanost-9(11)-ene-24S,25-diol (1) and 29-acetoxy-3beta-methoxyserrat-14-en-21alpha-ol (2) on the basis of spectroscopic evidence. Some of these triterpenes were tested for the inhibitory effect on DNA topoisomerase II activity. Compound 1 showed a slightly less potent inhibitory activity with an IC(50) value of 186 microM.     

Defining the molecular requirements for the selective delivery of polyamine conjugates into cells containing active polyamine transporters

Several N(1)-substituted polyamines containing various spacer units between nitrogen centers were synthesized as their respective HCl salts. The N(1)-substituents included benzyl, naphthalen-1-ylmethyl, anthracen-9-ylmethyl, and pyren-1-ylmethyl. The polyamine spacer units ranged from generic (4,4-triamine, 4,3-triamine, and diaminooctane) spacers to more exotic [2-(ethoxy)ethanoxy-containing diamine, hydroxylated 4,3-triamine, and cyclohexylene-containing triamine] spacers. Two control compounds were also evaluated: N-(anthracen-9-ylmethyl)-butylamine and N-(anthracen-9-ylmethyl)-butanediamine. Biological activities in L1210 (murine leukemia), alpha-difluoromethylornithine (DFMO)-treated L1210, and Chinese hamster ovary (CHO) and its polyamine transport-deficient mutant (CHO-MG) cell lines were investigated via IC(50) cytotoxicity determinations. K(i) values for spermidine uptake were also determined in L1210 cells. Of the series studied, the N(1)-benzyl-4,4-triamine system 6 had significantly higher IC(50) values (lower cytotoxicity) in the L1210, CHO, and CHO-MG cell lines. A cellular debenzylation process was observed in L1210 cells with 6 and generated "free" homospermidine. The size of the N(1)-arylmethyl substituent had direct bearing on the observed cytotoxicity in CHO-MG cells. The N(1)-naphthalenylmethyl, N(1)-anthracenylmethyl, and N(1)-pyrenylmethyl 4,4-triamines had similar toxicity (IC(50)s: approximately 0.5 microM) in CHO cells, which have an active polyamine transporter (PAT). However, this series had IC(50) values of >100 microM, 66.7 microM, and 15.5 microM, respectively, in CHO-MG cells, which are PAT-deficient. The observed lower cytotoxicity in the PAT-deficient CHO-MG cell line supported the premise that the conjugates use PAT for cellular entry. In general, moderate affinities for the polyamine transporter were observed for the N-arylmethyl 4,4-triamine series with their L1210 K(i) values all near 3 microM. In summary, the 4,4-triamine motif was shown to facilitate entry of polyamine conjugates into cells containing active polyamine transporters.     

HEC-cysteamine conjugates: influence of degree of thiolation on efflux pump inhibitory and permeation enhancing properties

Within the present study hydroxyethyl cellulose-cysteamine conjugates are investigated regarding biocompatibility, in situ gelling, permeation enhancing and efflux pump inhibitory properties. For this purpose, a series of concentrations of sodium periodate was prepared to oxidize HEC leading to ring opening of glucose subunits. The resulting polymers showing varying degrees of oxidation (DO) were then conjugated with cysteamine stabilized via reductive amination. Consequently, HEC-cysteamine conjugates with increasing degree in thiolation were obtained. Since the conjugates are positively charged, potency of cytotoxicity was tested by resazurin assay. In situ gelling properties of the conjugates were studied to investigate change of their viscosity due to inter- and/or intramolecular crosslinking via disulfide bonds. The influence of the presence of the conjugates on transport of rhodamine 123 and fluoresceinisothiocyanate-dextran 4 (FD4) representing model compounds for P-glycoprotein (P-gp) inhibition and permeation enhancing studies, respectively, across Caco-2 cell monolayers was determined. The conjugates showed a degree of thiolation in the range of 316-2158 μmol/g. Within 30 min, dynamic viscosity of the conjugate with the lowest degree of thiolation 0.5% (m/v) increased up to 300-fold. The conjugates showed a degree of thiolation-dependent increase in cytotoxicity but they all were found comparatively low cytotoxic. The addition of the conjugate with thiol group content of 1670 μmol/g resulted in the highest improvement in the transport of both rhodamine 123 and FD4 as compared to buffer control. Accordingly, the degree of thiolation strongly influences the properties of the conjugates and the modulation of the degree of thiolation could be exploited for development of various drug delivery systems.     

N1-substituent effects in the selective delivery of polyamine conjugates into cells containing active polyamine transporters

Several N(1)-arylalkylpolyamines containing various aromatic ring systems were synthesized as their respective HCl salts. The N(1)-substituents evaluated ranged in size from N(1)-benzyl, N(1)-naphthalen-1-ylmethyl, N(1)-2-(naphthalen-1-yl)ethyl, N(1)-3-(naphthalen-1-yl)propyl, N(1)-anthracen-9-ylmethyl, N(1)-2-(anthracen-9-yl)ethyl, N(1)-3-(anthracen-9-yl)propyl, and pyren-1-ylmethyl. The polyamine architecture was also altered and ranged from diamine to triamine and tetraamine systems. Biological activities in L1210 (murine leukemia), Chinese hamster ovary (CHO), and CHO's polyamine transport-deficient mutant (CHO-MG) cell lines were investigated via IC(50) cytotoxicity determinations. K(i) values for spermidine uptake were also determined in L1210 cells. The size of the N(1)-arylalkyl substituent as well as the polyamine sequence used had direct bearing on the observed cytotoxicity profiles. N(1)-Tethers longer than ethylene showed dramatic loss of selectivity for the polyamine transporter (PAT) as shown in a CHO/CHO-MG cytotoxicity screen. In summary, there are clear limits to the size of N(1)-substituents, which can be accommodated by the polyamine transporter. A direct correlation was observed between polyamine-conjugate uptake and cytotoxicity. In this regard, a cytotoxicity model was proposed, which describes a hydrophobic pocket of set dimensions adjacent to the putative PAT polyamine-binding site.     

雷公藤内酯醇-溶菌酶偶联物的性能研究及体外评价

目的 研究肾靶向前体药物雷公藤内酯醇-溶菌酶偶联物的理化性质.方法 采用HPLC法考察偶联物在37℃不同pH缓冲液、大鼠血浆及大鼠肾脏溶酶体溶液中的稳定性及细胞毒性.结果 体外稳定性试验表明,偶联物在37℃的不同pH缓冲液及大鼠血浆中稳定,在大鼠肾脏溶酶体溶液中可降解.偶联物能降低肾近端小管的细胞毒性,减少脂多糖引导的NO产生.结论 雷公藤内酯醇-溶菌酶偶联物可提高雷公藤内酯醇的肾靶向性,并可降低不良反应.     

Toxicity of the topoisomerase II inhibitors

Topoisomerase II inhibitors represent a broad class of antineoplastic agents with a wide spectrum of activity against malignancies. Topoisomerase II inhibitors include the anthracyclines, mitoxantrone and epipodophyllotoxins. Short-term toxicity includes myelosuppression and gastrointestinal toxicity. Long-term survivors are at risk of cardiac toxicity and secondary leukaemia. This article discusses these toxicities in detail, including administration of these agents to patients with hepatic and/or renal insufficiency, and the need for dose adjustments in selected patient populations.     

萘酰亚胺-多胺缀合物的合成及体外抗癌活性

合成了6个萘酰亚胺-多胺缀合物, 化合物的结构经元素分析、¹H NMR、¹³C NMR和MS确证。经MTT法对白血病细胞 (K562)、人乳腺癌细胞 (MB-231) 和前列腺癌细胞 (Ln cap cell) 进行了体外活性测试, 结果表明大多数化合物的体外抗肿瘤活性优于对照品氨萘非特 (amonafide), 其中化合物6d、6e和6f对正常人肝上皮细胞 (QSG-7701) 和肝癌细胞 (BEL-7402) 具有良好的选择性。     

DNA interaction and dual topoisomerase I and II inhibition properties of the anti-tumor drug prodigiosin.

Prodigiosin is a red pigment produced by Serratia marcescens with apoptotic activity. We examined the mechanism of action of this tripyrrole alkaloid, focusing on its interaction with DNA and its ability to inhibit both topoisomerase I and topoisomerase II. We also evaluated the DNA damage induced in cancer cell lines. Prodigiosin-DNA intercalation was analyzed using a competition dialysis assay with different DNA base sequences. Topoisomerase I and II inhibition was studied in vitro by a cleavage assay, and in cultured cells, by analysis of its ability to form covalent complexes. Furthermore, we analyzed DNA damage by pulse-field gel electrophoresis and by immunocytochemistry. Apoptosis inducing factor (AIF)/phospho-H2AX (p-H2AX) double labeling by confocal microscopy was performed to determine the possible implication of AIF in the prodigiosin-DNA damage. Finally, we studied the ability of this drug to induce copper-mediated DNA damage at different pH by a DNA cleavage assay. Our results demonstrate prodigiosin-DNA interaction in vitro and in cultured cells. It involves prodigiosin-DNA intercalation, with some preference for the alternating base pairs but with no discrimination between AT or CG sequences, dual abolition of topoisomerase I and II activity and, as consequence, DNA cleavage. Prodigiosin-DNA damage is independent of AIF. Furthermore, we found that copper-mediated cleavage activity is associated with pH (occurring at pH 6.8 rather than pH 7.4) and with the Cu(2+) ion concentration. These results indicate DNA a therapeutic target for prodigiosin and could explain the apoptosis mechanism of action induced by this antineoplastic drug.     

Synthesis and evaluation of novel amonafide–polyamine conjugates as anticancer agents

With the aim of upregulating antitumor efficacy and downregulating adverse effects, the amino group in the three‐position of amonafide aromatic ring was modified by coupling with different amine/polyamine motifs via two linkers. Two series of naphthalimide derivatives were designed and synthesized and evaluated for their antitumor properties in vitro and in vivo. The preliminary in vitro trials revealed that compounds with urea as the linker were not active, and the presence of aspirin elevated the potency of 6k against tumor cells, wound healing, and the protein expression of cyclic D1 and MMP9. The in vivo trials on three H22 tumor transplant models demonstrated that the combination of 6k and aspirin markedly improved the efficacy in terms of inhibitive effect, pulmonary metastasis, and extension of the life span. More importantly, the combination of 6k and aspirin displayed the reduced side‐effects compared to that of amonafide.     

Synthesis and DNA-binding properties of polyamine analogues

The synthesis of a series of novel polyamine analogues is reported. The DNA binding of these compounds and a variety of other polyamines were compared with their IC50 values against HeLa cell. There seemed to be no apparent correlation between the DNA binding and toxicity against HeLa cells.     

Influence of minor groove binders on the eukaryotic topoisomerase II cleavage reaction with 41 base pair model oligonucleotides

Summary This report deals with the cleavage reaction of calf thymus (CT) topoisomerase II with oligonucleotides containing one main cleavage site and adjacent binding sites for minor groove binders. The sequences of the oligonucleotides were derived from a pBR 322 sequence, which contains one main topoisomerase II cleavage site. The cleavage reaction was performed under increasing concentrations of minor groove binders and it showed characteristic inhibition dependences of topoisomerase II to the binding sites and to the binding length of the minor groove binders. The extension of the minor groove binder length on DNA from 4 to 10 base pairs (bp) by netropsin and bis-netropsin, respectively, causes a strong increase of the topoisomerase II cleavage inhibition. The same is observed by the introduction of a second minor groove binder sequence symmetrically positioned around the topoisomerase II main cleavage site. The combination of two different minor groove binders can lead to an increased topoisomerase II inhibition but also to a prevention of total inhibition as shown with chromomycin A₃ and distamycin A at concentrations of 0.1 and 0.25 M, respectively.     

The anticancer multi-kinase inhibitor dovitinib also targets topoisomerase I and topoisomerase II

Dovitinib (TKI258/CHIR258) is a multi-kinase inhibitor in phase III development for the treatment of several cancers. Dovitinib is a benzimidazole-quinolinone compound that structurally resembles the bisbenzimidazole minor groove binding dye Hoechst 33258. Dovitinib bound to DNA as shown by its ability to increase the DNA melting temperature and by increases in its fluorescence spectrum that occurred upon the addition of DNA. Molecular modeling studies of the docking of dovitinib into an X-ray structure of a Hoechst 33258–DNA complex showed that dovitinib could reasonably be accommodated in the DNA minor groove. Because DNA binders are often topoisomerase I (EC 5.99.1.2) and topoisomerase II (EC 5.99.1.3) inhibitors, the ability of dovitinib to inhibit these DNA processing enzymes was also investigated. Dovitinib inhibited the catalytic decatenation activity of topoisomerase IIα. It also inhibited the DNA-independent ATPase activity of yeast topoisomerase II which suggested that it interacted with the ATP binding site. Using isolated human topoisomerase IIα, dovitinib stabilized the enzyme-cleavage complex and acted as a topoisomerase IIα poison. Dovitinib was also found to be a cellular topoisomerase II poison in human leukemia K562 cells and induced double-strand DNA breaks in K562 cells as evidenced by increased phosphorylation of H2AX. Finally, dovitinib inhibited the topoisomerase I-catalyzed relaxation of plasmid DNA and acted as a cellular topoisomerase I poison. In conclusion, the cell growth inhibitory activity and the anticancer activity of dovitinib may result not only from its ability to inhibit multiple kinases, but also, in part, from its ability to target topoisomerase I and topoisomerase II.     

The antitumor triazoloacridone C-1305 is a topoisomerase II poison with unusual properties

C-1305 [S-[[3-(dimethylamino)propyl]amino]-8-hydroxy-6H-v-triazolo[4,5,1-de]acridin-6-one] is a triazoloacridone with excellent activity in colon cancer models. The mechanism of C-1305 is unknown, although similarities in the chemical structure between C-1305 and amsacrine suggest common cellular targets. Here, we report that C-1305 is a topoisomerase II poison that is able to induce cleavable complexes with topoisomerase II in vitro as well as in living cells. Even at optimal concentrations, C-1305 is a much weaker inducer of cleavable complexes than amsacrine. Because the cytotoxic activities of the two compounds after continuous drug exposure are comparable, these findings suggest that the low levels of cleavable complexes induced by C-1305 may be unusually toxic. In contrast to amsacrine, the cytotoxicity of C-1305 is strongly time-dependent, with at least 24 h of drug exposure required for optimal cytotoxicity. The p53 tumor suppressor is inactivated in the majority of human tumors, including colorectal cancers. We therefore compared the long-term cytotoxic effects of C-1305, amsacrine, and doxorubicin on human cell lines in which the p53 or p21 pathways have been specifically disrupted by targeted homologous recombination. Disruption of p53 and p21 had minor influence on the cytotoxicity of doxorubicin, whereas p53 but not p21 disruption was associated with increased resistance to amsacrine. In marked contrast, disruption of p53 and p21 was associated with increased sensitivity to C-1305. Taken together, our results show that exposure to C-1305 is accompanied by the formation of low levels of potent cleavable complexes that are selectively toxic toward tumor cells with defective p53 function.     

Influence of MCC II fraction and storage conditions on pellet properties

Microcrystalline cellulose II (MCC II) – a polymorph of commonly used MCC I – was introduced as new pelletization aid in wet-extrusion/spheronization leading to fast disintegrating pellets. Previous investigations suggested that pellet properties were influenced by the fraction of MCC II. Furthermore, it is unknown whether the storage conditions can affect the disintegration behavior. Therefore, the effects of MCC II fraction and the storage conditions on several pellet properties were investigated.MCC II-based pellets were prepared of pure MCC II or binary mixtures containing 10–50% (steps of 10%) MCC II as pelletization aid and theophylline, chloramphenicol or lactose. The pellets were characterized by their aspect ratio, equivalent diameter, water content, tensile strength, porosity as well as shrinking, and disintegration behavior and drug release according to their MCC II fraction. Furthermore, the pellets were stored at different relative humidities (0–97%rh), and the influence on their disintegration and drug release was investigated.With increasing MCC II fraction, the pellets became lager in size, decreased their porosity, and required higher water contents for spheronization. Moreover, the disintegration time increased and the disintegration itself was incomplete. Furthermore, the storage conditions had an impact on the disintegration properties of MCC II-based pellets. The disintegrating was affected irreversibly after storage at high humidity (80–97%rh) resulting in a slow drug release. Therefore, MCC II-based pellets need to be stored below 80%rh to secure a fast disintegration.A better knowledge of the properties of MCC II-based pellets was obtained providing a basis for a successful manufacturing and adequate storage of MCC II-based pellets prepared by extrusion/spheronization.     

Design, synthesis, and biological evaluation of indenoisoquinoline topoisomerase I inhibitors featuring polyamine side chains on the lactam nitrogen

The indenoisoquinolines are a class of noncamptothecin topoisomerase I inhibitors that display significant cytotoxicity in human cancer cell cultures. They offer a number of potential advantages over the camptothecins, including greater chemical stability, formation of more persistent cleavage complexes, and induction of a unique pattern of DNA cleavage sites. Molecular modeling has suggested that substituents on the indenoisoquinoline lactam nitrogen would protrude out of the DNA duplex in the ternary cleavage complex through the major groove. This indicates that relatively large substituents in that location would be tolerated without compromising biological activity. As a strategy for increasing the potencies and potential therapeutic usefulness of the indenoisoquinolines, a series of compounds was synthesized containing polyamine side chains on the lactam nitrogen. The rationale for the synthesis of these compounds was that the positively charged ammonium cations would increase DNA affinity through electrostatic binding to the negatively charged DNA backbone, and the polyamines might also facilitate cellular uptake by utilization of polyamine transporters. The key step in the synthesis involved the condensation of Schiff bases, containing protected amine side chains, with substituted homophthalic anhydrides, to afford cis-3-aryl-4-carboxy-1-isoquinolones. These isoquinolones were then converted to indenoisoquinolines with thionyl chloride. Although monoamines were much more potent than the lead compound, no significant increase in potency was observed through incorporation of additional amino groups in the side chain. However, one of the monoamine analogues, which features a bis(2-hydroxyethyl)amino group in the side chain, proved to be one of the most cytotoxic indenoisoquinoline synthesized to date, with a GI50 mean-graph midpoint (MGM) of 0.07 microM in the NIH human cancer cell culture screen, and topoisomerase I inhibitory activity comparable to that of camptothecin.