Anticancer specificity of some ellipticinium salts against human brain tumors in vitro.

Novel structure-activity relationships (SAR) distinct from known SAR for ellipticines have been revealed for certain ellipticinium salts. In particular, ellipticiniums such as the prototypical 9-methoxy-2-methylellipticinium (I- or OAc-) were found to be preferentially cytotoxic to the brain tumor cell line subpanel of the NCI 60 cell-line screening panel. Similar specificity also was apparent with 9-unsubstituted ellipticiniums, or others bearing 9-methyl or 9-chloro substituents. In contrast, 9-hydroxy-substituted ellipticiniums, as well as all nonquaternized ellipticines tested, were devoid of brain tumor specificity. Therefore, it did not appear that this unusual preference was correlated with the relative availability of redox cycling mechanisms, since redox cycling presumably is blocked in 9-methyl- and 9-chloroellipticiniums. Indeed, related investigations have indicated that the brain tumor specificity is mediated by preferential uptake and intracellular accumulation of the specific ellipticiniums. The present study further supports that the NCI in vitro "disease-oriented" primary screen can facilitate the discovery of novel, selectively cytotoxic leads for in vivo and mechanistic investigations.

Inhibition of nucleoside transport by nitrobenzylthioformycin analogs.

The formycin analogs of nitrobenzylthioinosine and nitrobenzylthioguanosine were synthesized and evaluated as nucleoside transport inhibitors. These analogs have a potential therapeutic advantage over their parent compounds in that their C-nucleosidic linkages prevent them from being degraded to the immunosuppressive agents, 6-mercaptopurine and 6-thioguanine. 7-[(4-Nitrobenzyl)-thio]-3-(beta-D-ribofuranosyl)pyrazolo[4,3- d]pyrimidine (NBTF) and 5-amino-7-[(4-nitrobenzyl)thio]-3-(beta-D- ribofuranosyl)pyrazolo[4,3-d]pyrimidine (NBTGF) were inhibitors of nucleoside transport in human erythrocytes and HL-60 leukemia cells. The IC50 value for nitrobenzylthioinosine, NBTF and NBTGF with 10% erythrocyte suspensions were 18, 18 and 40 nM respectively. Specific binding studies with [3H]NBTF yielded a Kd of 3.4 nM with erythrocytes, approximately 10-fold higher than values reported for nitrobenzylthioinosine. NBTF and nitrobenzylthioinosine bound to HL-60 cells with Kd values of 8.1 and 0.81 nM respectively. The octanol/water partition coefficients of nitrobenzylthioinosine, NBTF and NBTGF were 3.5, 3.2, and 2.8 respectively. NBTF could be expected to be equipotent with nitrobenzylthioinosine in whole blood where inhibitor concentrations of 10(-7) to 10(-6) M are required in order to saturate erythrocytic binding sites; hence, it may exhibit the advantages inherent in a C-nucleoside.

Characterization of adriamycin-resistant human breast cancer cells which display overexpression of a novel resistance-related membrane protein.

Development of multidrug resistance due to overexpression of P-glycoprotein (Pgp), a cell membrane drug efflux pump, occurs commonly during in vitro selections with adriamycin (Adr). Pgp-mediated drug resistance can be overcome by the calcium channel blocker verapamil (Vp), which acts as a competitive inhibitor of drug binding and efflux. In order to identify other mechanisms of Adr resistance, we isolated an Adr-resistant subline by selecting the human breast cancer cell line MCF-7 with incremental increases of Adr in the presence of 10 microgram/ml verapamil. The resultant MCF-7/AdrVp subline is 900-fold resistant to Adr, does not overexpress Pgp, and does not exhibit a decrease in Adr accumulation. It exhibits a unique cross-resistance pattern: high cross-resistance to the potent Adr analogue 3'-deamino-3'-(3-cyano-4-morpholinyl)doxorubicin, lower cross-resistance to the alkylating agent melphalan, and a sensitivity similar to the parental cell line to vinblastine. The levels of glutathione and glutathione S-transferase are similar in the parental line and the Adr-resistant subline. Topoisomerase II-DNA complexes measured by the potassium-sodium dodecyl sulfate precipitation method shows a 2-3 fold decrease in the resistant subline. The MCF-7/AdrVp cells overexpress a novel membrane protein with an apparent molecular mass of 95 kDa. Polyclonal antibodies raised against the P-95 protein demonstrate a correaltion between the level of expression and Adr resistance. Removal of Adr but not verapamil from the selection media results in a decline in P-95 protein levels that parallels a restoration of sensitivity to Adr. Immunohistochemistry demonstrates localization of the P-95 protein on the cell surface. The demonstration of high levels of the protein in clinical samples obtained from patients refractory to Adr suggests that this protein may play a role in clinical drug resistance.

Neurotoxicity and dermatotoxicity of cyanomorpholinyl adriamycin.

The highly lipophilic cyanomorpholinyl adriamycin (CMA) is the most potent antineoplastic anthracycline yet described. CNS distribution and toxicity were examined after i.v. administration of CMA to mice. At doses greater than or equal to 0.1 mg/kg, a neurotoxic syndrome including ataxia, hypokinesia, and tremors appeared. At doses of less than or equal to 0.05 mg/kg, which have been reported to be antineoplastic, no neurotoxicity was observed. On histopathologic examination, no changes were observed in the brain, spinal cord, or dorsal root ganglia. Unlike adriamycin (ADR), which rapidly appears in the nuclei of several tissues, CMA showed no fluorescence, suggesting a different cellular microcompartmentalization. The i.d. injection of CMA disclosed a 200-fold increase in toxicity compared with that of adriamycin. In comparisons of CMA and ADR, neurotoxicity and cardiotoxicity occurred equally only at higher doses; however, the dermatotoxicity and antineoplastic activity of CMA were increased several hundred-fold.

Effect of morpholinyladriamycin analogs and adriamycin on the activities of DNA polymerase alpha and RNA polymerase II of chicken leukemia cells.

The new adriamycin (ADR) derivatives, 3'-deamino-3'-(4"-morpholinyl)adriamycin (MRA) and 3'-deamino-3'-(3"-cyano-4"-morpholinyl)-adriamycin (MRA-CN), were studied, in comparison with ADR, for their inhibitory effects on DNA and RNA syntheses in vitro using isolated DNA and RNA polymerases from both Escherichia coli and chicken (myeloblastosis) leukemia cells. Under standard assay conditions, MRA and ADR demonstrated a similar inhibitory effect on the enzymes, whereas MRA-CN showed a slightly greater inhibitory activity than ADR or MRA at low drug concentrations, but with the inhibitory effect plateauing when drug concentration reached 10 microM. Both the A and B forms of the MRA-CN diastereoisomers were effective as inhibitors. Kinetic studies of the inhibition showed that unlike ADR, MRA-CN inhibition of DNA or RNA synthesis could not be reversed by increasing DNA-template concentration in the reaction mixture. Whereas ADR or MRA relaxed completely 1 microgram of pBR322 supercoiled DNA at a drug concentration of 15 microM, MRA-CN, at 60 microM, produced a mixture of intermediate relaxation forms of the DNA. A complete relaxation was achieved at 300 microM MRA-CN. Both DNA relaxation and fluorescence spectroscopic studies indicated that DNA-drug interactions occurred in the following order: ADR (or MRA) greater than MRA-CN greater than N-trifluoroacetyl-ADR-14-O-hemiadipate (a DNA-nonbinding anthracycline).

N-(cyanomethyl)- and N-(2-methoxy-1-cyanoethyl)anthracyclines and related carboxyl derivatives.

Treatment of doxorubicin with formaldehyde and NaCN afforded the N-(cyanomethyl) derivative as a stable alpha-cyanoamine with but moderate antitumor activity in mice, although it was prototypal to the intensely potent alpha-cyanomorpholine derivative. 2-Methoxyacetaldehyde and NaCN afforded the N-(2-methoxy-1-cyanoethyl) derivative as an open-chain analogue of the cyanomorpholine. This analogue underwent rapid hydrolysis to doxorubicin and appeared to act as a prodrug, giving increased antitumor efficacy although with decreased potency. N-(Carboxymethyl)daunorubicin was a highly water-soluble but inactive analogue, synthesized by N-alkylation with ethyl iodoacetate and saponification. The similar N-alkylation of N-(cyanomethyl) daunorubicin demonstrated the combining of N-alkyl chains having different functional substituents.

N-(2-hydroxyethyl)doxorubicin from hydrolysis of 3'-deamino-3'-(3-cyano-4-morpholinyl)doxorubicin.

The susceptibility of 3'-deamino-3'-(3-cyano-4-morpholinyl)doxorubicin to hydrolysis at pH 7, 4, and 2 has been compared with that of the typically stable morpholine analogue. At pH 7, 74% of the cyanomorpholine was unchanged after 24 h at room temperature, but at pH 2 only 10% remained. Products identified were aglycon (8%) and N-(2-hydroxyethyl)doxorubicin (7%). Most of the losses were to unidentified polar products not eluted from HPLC. Authentic hydroxyethyl was synthesized from doxorubicin by reductive alkylation with glycolaldehyde. Antitumor potency was comparable to that of doxorubicin rather than of cyanomorpholine.

Effects of 3'-deamino-3'-(3-cyano-4-morpholinyl)doxorubicin and doxorubicin on the survival, DNA integrity, and nucleolar morphology of human leukemia cells in vitro.

The potential mechanisms of the extremely potent anthracycline analogue 3'-deamino-3'-(3-cyano-4-morpholinyl)doxorubicin (MRA-CN) have been compared with those of doxorubicin (DOX) by examination of drug effects on colony formation, macromolecular synthesis, DNA integrity, and ultrastructure of human leukemia cells in vitro. Following a 1-h exposure, MRA-CN was found to be 1400-fold more cytocidal than DOX which correlated with the drugs' inhibitory effects on DNA and total RNA synthesis. Treatment with MRA-CN resulted in a dose-dependent production of DNA interstrand cross-links as quantified by alkaline elution. One-h treatments with DOX or 3'-deamino-3'-(4-morpholinyl) doxorubicin (the non-cyano-containing analogue of MRA-CN) produced no DNA-DNA cross-links; rather they produced protein-concealed DNA single-strand breaks. After removal of MRA-CN, the DNA of KBM-3 cells displayed time-dependent fragmentation as indicated by rapid DNA filter elution during the pH 10 lysis step which preceded pH 12 elution. Within 4 h of MRA-CN exposure (10 nM, 1 h), 50% of the cellular DNA was in the lysis fraction. By 24 h, all the cellular DNA was in this fraction. MRA-CN (10 nM), 3'-deamino-3'-(4-morpholinyl)doxorubicin (1 microM), and actinomycin D (1 microM), but not DOX (3 mircroM), each produced distinctive nucleolar macrosegregation, indicating an effect on rRNA synthesis. The alpha-CN substituent on the morpholinyl moiety of MRA-CN appears to be responsible for the unique antitumor potency of this anthracycline. Nucleolar macrosegregation is probably associated with the morpholinyl moiety and is independent of the alpha-CN substituent.

New cyanomorpholinyl byproduct of doxorubicin reductive alkylation.

Previously we reported that reductive alkylation of doxorubicin with 2,2'-oxybis[acetaldehyde] and NaBH3CN to form the 4''-morpholinyl derivative also gave the intensely potent 3''-cyano-4''-morpholinyl as a byproduct, by addition of CN- to an iminium intermediate in place of hydride. We now find that sugar 4'-OH is a third nucleophile that can add to the iminium intermediate in this reaction. Bridging of the 4'-OH to the morpholine ring at C.5'' formed a novel byproduct with an oxazolidino ring fused to the sugar and morpholine. The new product was minor at neutral pH but predominant at an acidic pH. When tested against tumors in mice it was 4-6 times more potent than doxorubicin. Hence, in comparison with the 3''-cyano-4''-morpholinyl, potency was reduced up to 100-fold by the O bridge. Analytical HPLC showed the presence of three of the four possible diastereoisomers, and two were isolated. The diastereoisomers appeared to differ in stability. In vitro tests suggested that biological potency varied inversely with stability.

Dissociation of antitumor potency from anthracycline cardiotoxicity in a doxorubicin analog.

The search for new congeners of the leading anticancer drug doxorubicin has led to an analog that is approximately 1000 times more potent, noncardiotoxic at therapeutic dose levels, and non-cross-resistant with doxorubicin. The new anthracycline, 3'-deamino-3'-(3-cyano-4-morpholinyl)doxorubicin (MRA-CN), is produced by incorporation of the 3' amino group of doxorubicin in a new cyanomorpholinyl ring. The marked increase in potency was observed against human ovarian and breast carcinomas in vitro; it was not accompanied by an increase in cardiotoxicity in fetal mouse heart cultures. Doxorubicin and MRA-CN both produced typical cardiac ultrastructural and biochemical changes, but at equimolar concentrations. In addition, MRA-CN was not cross-resistant with doxorubicin in a variant of the human sarcoma cell line MES-SA selected for resistance to doxorubicin. Thus antitumor efficacy was dissociated from both cardiotoxicity and cross-resistance by this modification of anthracycline structure.

Comparative cytotoxicities of various morpholinyl anthracyclines.

A series of quinone- and sugar-modified analogs of adriamycin have been tested for growth inhibition of adriamycin-sensitive (P388/S) and -resistant (P388/ADR) sublines of P388 murine leukemia cells in vitro. P388/ADR is less resistant to analogs of adriamycin containing either a 3'-deamino-3'-(4"-morpholinyl) group, MRA; or a -(3"-cyano-4"-morpholinyl) group, MRA-CN, than to adriamycin. However, MRA-CN was the most potent growth inhibitor of either subline. This potency is reduced by either modification of the quinone unit with a 5-imino substituent or restriction of the cyano-morpholinyl ring by an oxygen bridge to the daunosamine sugar. The calcium antagonist verapamil substantially increases the cytotoxicity of adriamycin to P388/ADR but has no appreciable effect on the cytotoxicity of either MRA or MRA-CN. The results suggest that increased uptake and retention by both MRA and MRA-CN may contribute to their increased cytotoxicity, but that the intense potency of the cyano-morpholinyl analogs must be due to other unique properties of these compounds.

Metabolic and toxicologic study of an artificial sweetener, oxime V.

Metabolic disposition and subchronic oral toxicologic studies were conducted on a new synthetic sweetener, Oxime V. Based on radioactivity assay, the compound was readily absorbed and metabolized. Excretion was nearly quantitative 48 hours after dosing the rat, dog, and rhesus monkey. The major metabolites were formed by oxidation and reduction of the cyclohexadiene ring, oxidation of the aldoxime and dimethyl ether moieties followed by conjugation with glycine, thiomethylation of the ring, and O-glucuronidation of the aldoxime. A two-month feeding study was conducted with male adult rats. The average consumption of Oxime V was 396.5 mg/kg per day by rats fed a diet containing 0.6% of the test compound. No treatment related histopathologic lesion was observed in the liver, kidney, spleen, and testes. The liver weight relative to the body weight and serum bilirubin level were increased.

Anthracycline mechanisms in analogue selection.

Several simple test methods that revealed alterations in mechanisms of action proved to be useful in the selection of three new anthracycline analogues that are currently in preclinical development. 5-Iminodaunorubicin is a quinone-modified analogue, and the resultant suppression of quinone redox cycling appears to correlate with diminished cardiotoxicity rather than with any effect on antitumour activity. N,N-Dibenzyldaunorubicin is an inactive prodrug requiring metabolic activation, a process that appears to give some selectivity of action leading to improved activity. The cyanomorpholino derivative of doxorubicin shows an intense potency against tumours that is not encountered in other closely-related analogues, indicating a highly specific mode of action as yet unidentified. Results with these examples suggest that simple tests related to mechanisms of action may be more useful for analogue selection than extended tests to define antitumour activity.

Electrocardiographic and transmembrane potential effects of 5-iminodaunorubicin in the rat.

5-Iminodaunorubicin (5-ID) is a quinone-modified anthracycline that retains antitumor activity but lacks the usual redox-cycling effects of quinoid agents. As a test for decreased cardiotoxicity, we have compared the dose- and time-dependent effects of multiple doses of 5-ID and doxorubicin (DXR) on the rat electrocardiogram (ECG) using a signal-averaging process and have related the ECG changes induced by 5-ID to transmembrane potential alterations in myocardial preparations isolated from treated rats. 5-ID was studied at dose levels of 16, 4, and 1 mg/kg, while DXR was given at 4, 2, and 1 mg/kg. At the high- and medium-dose levels, both agents produced widening of the QRS complex, increased R- and S-wave voltage, and prolonged the Q alpha T interval. The QRS widening reversed in all surviving rats, whereas Q alpha T prolongation was reversible with 5-ID but irreversible with DXR. At the lowest dose, 5-ID had no effect on the ECG until the end of treatment. Microelectrode studies on single cells showed that QRS widening occurring with 5-ID treatment was related to a decrease in the maximum rate of depolarization (Vmax) and that Q alpha T prolongation resulted from an increase in the duration of the action potential. Electron microscopic examination showed that although these toxic changes could not be related to specific morphological alterations, in general, the more severe the electrophysiological change, the greater the ultrastructural change. The most consistent ECG change was Q alpha T prolongation. Using this parameter as a marker for cardiotoxicity, 5-ID was about 4 to 5 times less cardiotoxic than was DXR at high- and medium-dose levels and was noncardiotoxic (i.e., below a threshold for cardiotoxicity) compared with DXR at 1 mg/kg over 20 (DXR) to 35 (5-ID) treatments. The decrease in cardiotoxicity relative to DXR is consistent with previous findings that quinone redox cycling is suppressed in 5-ID. However, the ECG and transmembrane potential effects that we identified at elevated doses of 5-ID can be associated with toxic changes in cardiac cell membranes. Therefore, membrane changes other than those due to quinone redox cycling and, presumably, lipid peroxidation must underlie the electrophysiological changes and structural modifications observed with 5-ID in this study. We believe that 5-ID is a useful mechanistic probe in anthracycline cardiotoxicity studies as well as being of obvious interest for clinical trials.

Intensely potent morpholinyl anthracyclines.

3'-Deamino-3'-(3-cyano-4-morpholinyl)doxorubicin is a new analogue that is 100 to 1000 times more potent than doxorubicin against tumors in cell culture or in mice, that is active by intraperitoneal, intravenous, or oral dosing, and that does not produce chronic myocardial lesions in mice. This analogue was encountered in studies on the reductive alkylation of doxorubicin and daunorubicin with 2,2' - oxybis [acetaldehyde], which constructs a morpholino ring incorporating the amino N. The morpholinyl nitrile byproducts are separated by virtue of their nonbasicity from the expected morpholino derivatives. The 13-dihydro and 5-imino derivatives are also described in this important new class of anthracyclines.

Metabolic disposition of 5-iminodaunorubicin in the rat.

We determined the metabolic disposition of 5-iminodaunorubicin (IMD) in rats receiving acute doses of IMD (16 and 4 mg/kg) i.v., i.p., and p.o. Major compounds found in plasma, liver, heart, lung, and brain of the rats receiving the higher dose, i.v. or i.p., were IMD and 5-imino-13-dihydrodaunorubicin. The aglycones, 5-iminodaunorubicinone and 5-imino-13-dihydrodaunorubicinone, were minor metabolites. No deoxyaglycones of IMD were detected in any tissue. We could not detect daunorubicin or its metabolites indicating IMD was not a prodrug of daunorubicin. Highest levels of IMD and metabolites were found in lung, liver, and heart, and lowest levels were found in the plasma and brain. Plasma levels of IMD after the higher i.v. dose decayed in a biphasic manner, and we calculated alpha-phase and beta-phase halftimes of decay of 1.4 and 10 hr, respectively. Patterns of distribution of IMD and metabolites were very similar following i.v. and i.p. treatments, except for higher amounts of IMD and metabolites in the liver after the latter route. A p.o. dose of IMD (16 mg/kg) yielded plasma levels of IMD that were only about 20% of those found by the parenteral routes. Summations of all compounds in all tissues at all times after this treatment yielded less than 2% of the corresponding totals found by the other routes. Results obtained following IMD (4 mg/kg) by the three routes generally confirmed conclusions derived from the studies at the higher dose, and we found an approximate linear dose relationship between the results from the two studies. Our inability to detect the formation of deoxyaglycone metabolites of IMD in vivo is consistent with earlier in vitro findings that IMD does not participate in the oxygen-cycling phenomenon typical of daunorubicin and doxorubicin to form drug and oxygen radicals and deoxyaglycone metabolites.

Doxorubicin cardiotoxicity in the rat: comparison of electrocardiogram, transmembrane potential, and structural effects.

We have evaluated the dose-, schedule-, and time-dependent effects of doxorubicin (DXR) on the rat electrocardiogram (ECG) and have related ECG alterations to cellular transmembrane potential (TMP) changes and ultrastructural changes in preparations isolated from DXR-treated animals. DXR was administered intraperitoneally at 1, 2, and 4 mg/kg (all five times per week) or 5 mg/kg once per week up to a cumulative dose of 20 mg/kg, or 2 mg/kg daily for 5 days for a cumulative dose of 10 mg/kg. Posttreatment deaths were due to acute toxicity (bone marrow suppression) or congestive cardiomyopathy (8-14 weeks after the end of dosing). The most consistent ECG changes observed with DXR treatment (greater than 10 mg/kg cumulative dose) were a reversible prolongation of the QRS complex and a progressive lengthening of the Q alpha T interval; changes in R-, S-, and T-wave voltages were more variable. ECG toxicity was more pronounced when the drug was given on a weekly schedule. On the cellular level, DXR treatment led to a decrease in Vmax, with a slight increase or no change in resting potential, and a marked prolongation in action potential duration at the 50% and 75% repolarization levels. The ECG and TMP changes were accompanied by ultrastructural changes that increased in severity during the posttreatment period. Our data further support the usefulness of the rat ECG in anthracycline cardiotoxicity studies.

Metabolic disposition of N,N-dibenzyldaunorubicin in the rat.

The more efficacious and less cardiotoxic analogue of daunorubicin, N,N-dibenzyldaunorubicin (B2D), was found to be metabolized in rats by stepwise debenzylation that was superimposed on the known anthracycline metabolism via 13-ketone reduction and deglycosidation. Using high-pressure liquid chromatography for resolution and fluorescence for detection, we observed a series of metabolites in plasma, liver, heart, muscle, and lungs of rats receiving 10 mg B2D per kg, i.v., i.p., and p.o. Rats receiving 40 mg B2D per kg, i.v., died immediately, but this dose given p.o. was not lethal during 24 hr. Patterns of B2D and metabolites varied quantitatively with tissue and route of administration. Rat liver perfusion studies indicated extensive metabolism of B2D compared with limited metabolism of doxorubicin. These observations were consistent with an observed major first-pass effect on B2D in intact rats given B2D p.o. The predominant metabolites of B2D were the glycosidic derivatives, N-benzyldaunorubicin, daunorubicin, and their 13-dihydro derivatives. These metabolites of B2D had exhibited activity against mouse leukemia P388 as did B2D and were active in in vitro tests in which B2D was essentially inactive. These results indicate that B2D acts as a prodrug for a series of active metabolites. Conversion of B2D to these metabolites was relatively more efficient after p.o. administration than following i.v. or i.p. treatments.

Further studies on the generation of reactive oxygen species from activated anthracyclines and the relationship to cytotoxic action and cardiotoxic effects.

The relative ease of generation of reactive oxygen species from a series of reductively activated aglycone and sugar modified anthracyclines was compared by the extents of single strand scission in supercoiled PM2-covalently closed circular (CCC)-DNA. The electrochemical properties of these agents were used as a quantitative measure of the ease of reduction and subsequent reoxidation of the reduced species. The relationship of these processes to various biological properties of the anthracyclines, in particular to the measured cardiotoxicity of the drugs, was examined. An attempt was made to identify those structural changes which ameliorate the cardiotoxic effects measured in other test systems while permitting the expression of antitumor properties.