Schedule-dependent activity of irinotecan plus BCNU against malignant glioma xenografts

PURPOSE

To further evaluate the activity of irinotecan (CPT-11) plus 1,3-bis-(chloroethyl)-1-nitrosourea (BCNU) in the treatment of central nervous system tumor-derived xenografts in athymic nude mice.

METHODS

We report studies evaluating the schedule-dependence of this regimen in the treatment of the malignant glioma xenograft D-54 MG.

RESULTS

The combination of BCNU and CPT-11 showed the highest enhancement index (2.0-3.3) when BCNU was given on day 1 and CPT-11 was given on days 1-5 and 8-12. Delay of CPT-11 administration to day 3 or day 5 substantially decreased activity with enhancement indices of 1.6-1.8 and 0.6-1.0, respectively. Delay of BCNU administration to day 8 also reduced the CPT-11 activity with enhancement indices of 1.2-1.4.

CONCLUSIONS

These results suggest that the presence of a BCNU-induced adduct or possibly crosslink prior to administration of CPT-11 is critical for enhanced activity. Although the mechanism of this enhancement is not currently known, a phase I trial of CPT-11 plus BCNU for adults with recurrent malignant glioma based on these results is in progress.

Schedule-dependent activity of temozolomide plus CPT-11 against a human central nervous system tumor-derived xenograft

Temozolomide, an imidazole tetrazinone, and CPT-11, a camptothecin derivative, have previously been shown to have anti-central nervous system tumor activity in laboratory and clinical studies. The current experiments were designed to evaluate the activity of temozolomide plus CPT-11 against a malignant glioma-derived xenograft, D-54 MG, growing s.c. in athymic nude mice. The initial schedule of i.p. drug administration was temozolomide at 0.1 LD10 on day 1 and CPT-11 at 0.1 LD10 on days 1-5 and 8-14. The combination of these two agents produced greater than additive activity against D-54 MG. This enhanced activity was maintained when the initial administration of CPT-11 was delayed to day 3 or day 5. However, when CPT-11 was administered first on day 1 using 0.5 LD10 (for the single dose schedule) followed by temozolomide (0.1 LD10) 5 h, 3 days, or 5 days later, the enhancement of activity was substantially reduced. These results demonstrate that the combination of temozolomide plus CPT-11 displays a schedule-dependent enhancement of antitumor activity, suggest a mechanistic explanation for the enhanced activity, and provide the rationale for a Phase I trial of this regimen.

Therapeutic efficacy of vinorelbine against pediatric and adult central nervous system tumors

PURPOSE

The activity of vinorellbine, a new semisynthetic vinca alkaloid, was evaluated against a battery of human tumor xenografts derived from adult and pediatric CNS malignancies.

METHODS

Tumors included adult high-grade gliomas (D-54 MG, D-245 MG), childhood high-grade gliomas (D-212 MG, D-456 MG), medulloblastomas (D-341 MED, D-487 MED), ependymomas (D-612 EP, D-528 EP), and a mismatch repair-deficient procarbazine-resistant glioma [D-245 MG (PR)]. Tumors were grown subcutaneously in athymic nude mice and vinorelbine was administered at a dose of 11 mg/kg on days 1, 5, and 9. Additionally, vinorelbine was also administered in combination with BCNU against D-54 MG.

RESULTS

Vinorelbine produced statistically significant growth delays in D-456 MG, D-245 MG, and D-245 MG (PR). No statistically significant growth delays were observed in D-54 MG, D-487 MED, D-212 MG, D-528 EP, D-341 MED or D-612 EP. The antitumor effects of the vinorelbine/BCNU combination were additive. Growth delays observed in the procarbazine-resistant line [D-245 MG (PR)] were greater than twofold the delays seen in the parent line (D-245 MG). Vincristine was equally potent against D-245 MG and D-245 MG (PR). Taxol demonstrated little activity against D-245 MG but produced 32- and 18-day growth delays in D245 MG (PR).

CONCLUSIONS

These studies indicate that vinorelbine possesses antitumor activity against several glioma tumor xenografts with marked activity in a mismatch repair deficient-tumor.

Enhancement of irinotecan (CPT-11) activity against central nervous system tumor xenografts by alkylating agents

Two major obstacles in the treatment of patients with central nervous system malignancies are drug resistance and host toxicity. The goal of combination chemotherapy is to achieve therapeutic effects that are more favorable than using a single drug alone, but without an increase in normal organ toxicity. The study reported here examined the combination of a topoisomerase I inhibitor, irinotecan (CPT-11), with three different alkylating agents: 1,3-bis(2-chloroethyl)-1-nitrosourea, busulfan, and cyclophosphamide. We evaluated the antitumor effects of these three combinations against a panel of human tumor xenografts derived from central nervous system malignancies, including adult high-grade gliomas (D-54 MG, D-245 MG) and a childhood ependymoma (D-612 EP). In replicate experiments, the alkylating agents were given on day 1 in doses varying from 10% to 75% of the dose lethal to 10% of the animals, and CPT-11 was given on days 1-5 and 8-12 in doses varying from 10% to 100% of the dose lethal to 10% of the animals. The antitumor effects of the various combinations ranged from less than additive (7.61 days below additive with 0.5 CPT-11 + 0.75 cyclophosphamide in D-54 MG) to statistically significant (P < 0.001) supraadditive effects (18.80 days above additive with 0.5 CPT-11 + 0.5 1,3-bis(2-chloroethyl)-1-nitrosourea in D-54 MG). These studies show that the combination of the topoisomerase inhibitor CPT-11 and alkylating agents may increase the antitumor effect in some cases well above additive with no increase in host toxicity (0/10 deaths in both experiments cited above) and should be considered for combination chemotherapy of central nervous system malignancies.

Characterization of the mechanisms of busulfan resistance in a human glioblastoma multiforme xenograft

Busulfan is an alkylating agent commonly used in the treatment of chronic myelogenous leukemia and in combination with cyclophosphamide in preparation for allogeneic bone marrow transplantation. Serial treatment of a childhood high-grade glioma xenograft (D-456 MG) with busulfan resulted in a busulfan-resistant xenograft, D-456 MG(BR). Cross-resistance to 1,3-bis(2-chloroethyl)-1-nitrosourea was seen but not resistance to cyclophosphamide or CPT-11. Cytoplasmic levels of glutathione in D-456 MG(BR) were approximately one-half those found in D-456 MG. This depletion could not be explained by levels of glutathione-S-transferase, or by amplification, rearrangement, or increased levels of transcript of gamma-glutamylcysteine synthetase. Furthermore, depletion of glutathione in D-456 MG did not alter busulfan activity. Quantitation of busulfan levels in D-456 MG and D-456 MG(BR) xenografts following treatment of mice at the dose lethal to 10% of the animals demonstrated that significantly lower levels of drug were achieved in D-456 MG(BR). These studies suggest that alterations in drug transport or metabolism of busulfan may play a role in the resistance of D-456 MG(BR) to this alkylator.

L-amino acid oxidase (LOX) modulation of melphalan activity against intracranial glioma

These studies evaluated the efficacy of sequential pretreatment with L-amino acid oxidase (LOX) and LOX antiserum in the modulation of melphalan activity against intracranial glioma in athymic nude mice. LOX produced statistically significant (P < 0.01) depletion of the large neutral amino acids isoleucine, leucine, methionine, phenylalanine, tyrosine, and valine in murine plasma at doses of 100 and 200 micrograms administered intravenously. Polyclonal anti-LOX antibody was successfully produced in mice, rabbits, and goats subsequent to immunization with LOX. Staphylococcal protein A-purified rabbit anti-LOX serum inhibited approximately 50% of LOX activity in vitro relative to control samples. This antiserum was used in vivo to inactivate LOX after it had depleted the large neutral amino acids, thereby preventing LOX-mediated catabolism of melphalan. Inoculation of three mice with rabbit anti-LOX serum after the treatment with LOX (100 micrograms) reduced LOX activity by 100%, 89%, and 100% at 6 h compared with reductions of 80%, 59%, and 52% over the same period in animals receiving LOX alone. In three separate studies using groups of eight to ten mice bearing intracranial human glioma xenografts, pretreatment with LOX followed by anti-LOX serum increased the antitumor activity of melphalan as compared with treatments with melphalan plus LOX, melphalan plus anti-LOX serum, or melphalan alone.

Therapeutic efficacy of the topoisomerase I inhibitor 7-ethyl-10-(4-[1-piperidino]-1-piperidino)-carbonyloxy-camptothecin against pediatric and adult central nervous system tumor xenografts

Therapy of patients with malignant central nervous system tumors is frequently unsuccessful, reflecting limitations of current surgical, radiotherapeutic, and pharmacotherapeutic treatments. The camptothecin derivative irinotecan (CPT-11) has been shown to possess antitumor activity in phase II trials for patients with carcinoma of the lung, cervix, ovary, colon, or rectum and for patients with non-Hodgkin's lymphoma. The current study was designed to test the efficacy of the drug against a panel of human tumor xenografts derived from adult and pediatric central nervous system malignancies. Tumors included childhood high-grade gliomas (D-212 MG, D-456 MG), adult high-grade gliomas (D-54 MG, D-245 MG), medulloblastomas (D341 Med, D487 Med), ependymomas (D528 EP, D612 EP), and a rhabdomyosarcoma (TE-671), as well as sublines with demonstrated resistance to busulfan (D-456 MG (BR)), cyclophosphamide (TE-671 CR), procarbazine (D-245 MG (PR)) or melphalan (TE-671 MR), growing subcutaneously and intracranially in athymic nude mice. In replicate experiments, CPT-11 was given at a dosage of 40 mg/kg per dose via intraperitoneal injection in 10% dimethylsulfoxide on days 1-5 and 8-12, which is the dosage lethal to 10% of treated animals. CPT-11 produced statistically significant (P < 0.001) growth delays in all subcutaneous xenografts tested, including those resistant to busulfan, cyclophosphamide, procarbazine, and melphalan, with growth delays ranging from 21.3 days in D487 Med to 90+ days in several tumor lines. Further, tumor regression was evident in every treated animal bearing a subcutaneous tumor, with some xenografts yielding complete tumor regression. Statistically significant (P < 0.001) increases in survival were demonstrated in the two intracranial xenografts-D341 EP (73.0% increase) and D-456 MG (114.2% increase)-treated with CPT-11. These studies demonstrate that, of over 40 drugs evaluated in this laboratory, CPT-11 is the most active against central nervous system xenografts and should be advanced to clinical trial as soon as possible.

Enhancement of melphalan activity by inhibition of DNA polymerase-alpha and DNA polymerase-beta

Our previous studies exploring melphalan resistance in the human rhabdomyosarcoma xenograft TE-671 MR revealed elevation of DNA polymerase-alpha and DNA polymerase-beta. The present study evaluated the alteration of melphalan activity in TE-671 (melphalan-sensitive) and TE-671 MR (melphalan-resistant) subcutaneous xenografts in nude mice after DNA polymerase-alpha was inhibited using aphidicolin glycinate (AG) and DNA polymerase-beta was inhibited using dideoxycytidine (DDC). Administration of AG or DDC did not produce toxicity or demonstrate antineoplastic activity when given alone. AG (90 mg/m2) enhanced the activity of melphalan against TE-671, with growth delays increasing by 8.4, 15.8, and 21.2 days over the regimen with melphalan only. AG (180 mg/m2) only modestly increased melphalan activity against TE-671 MR, with the growth delays increasing from 9.6 and 12.1 days using melphalan alone to 12.1 and 14.5 days using melphalan plus AG. AG (180 mg/m2) plus melphalan (the dose lethal to 10% of animals) produced greater weight loss compared with melphalan alone, whereas DDC plus melphalan produced no additional toxicity. DDC modestly enhanced the activity of melphalan plus AG against TE-671 MR. AG plus O6-benzylguanine did not increase the activity of 1,3-bis(2-chloroethyl)-1-nitrosourea against TE-671 or TE-671 MR. AG (90 mg/m2 and 180 mg/m2) inhibited DNA polymerase-alpha to 80% and 72% of control in TE-671 and 64% and 37% in TE-671 MR, and DDC inhibited DNA polymerase-beta to 59% in TE-671 and 48% in TE-671 MR. These results suggest a role for AG-mediated enhancement of melphalan activity, particularly in the treatment of newly diagnosed, melphalan-sensitive tumors.

Flunarizine enhancement of melphalan activity against drug-sensitive/resistant rhabdomyosarcoma

Flunarizine, a diphenylpiperazine calcium channel blocker, is known to increase tumor blood flow. It also interferes with calmodulin function, repair of DNA damage and drug resistance associated with P-glycoprotein. Flunarizine was tested for its ability to modulate either cyclophosphamide- or melphalan-induced growth delay for a drug-resistant rhabdomyosarcoma xenograft (TE-671 MR) and the drug-sensitive parent line (TE-671), in which P-glycoprotein is not involved in the mechanism of drug resistance. Tumour blood flow was increased by 30% after a flunarizine dose of 4 mg kg-1, but no modification in growth delay was induced by melphalan (12 mg kg-1). In contrast, a 60 mg kg-1 dose of flunarizine had no effect on tumour blood flow, but the same dose created significant enhancement in melphalan-induced tumour regrowth delay in both tumour lines. The dose-modifying factor for flunarizine as an adjuvant to melphalan was approximately 2 for both tumour lines. Although blood flow measurements were not performed with the combination of flunarizine and melphalan, the results from flunarizine alone suggested that augmentation of melphalan cytotoxicity is not mediated by changes in blood flow. In contrast, flunarizine did not affect drug sensitivity to cyclophosphamide in groups of animals bearing the drug-sensitive parent tumour line. These results suggest that the mechanism of drug sensitivity modification by flunarizine is not related to modification of tumour blood flow, but may be mediated by modification of transport mechanisms that are differentially responsible for cellular uptake and retention of melphalan as compared with cyclophosphamide.

The effect of L-amino acid oxidase on activity of melphalan against an intracranial xenograft

We have previously shown that diet restriction-induced depletion of large neutral amino acids (LNAAs) in murine plasma to 46% of control significantly enhances intracranial delivery of melphalan without enhancing delivery to other organs. Studies have now been conducted to determine whether more substantial LNAA depletion could further enhance intracranial delivery of melphalan. Treatment with L-amino acid oxidase (LOX) significantly depleted murine plasma LNAAs: phenylalanine, leucine, and tyrosine (> 95%); methionine (83%); isoleucine (70%); and valine (46%). Experiments evaluating the intracellular uptake of melphalan and high-pressure liquid chromatography quantitation of melphalan metabolites revealed, however, that melphalan is rapidly degraded in the presence of LOX, and that the timing of the administration of melphalan following the use of LOX to deplete LNAAs is crucial. Conditions were found under which LOX-mediated degradation of melphalan was minimized and LNAA depletion was maximized, resulting in a potentiation of the antitumor effect of melphalan on human glioma xenografts in nude mice. Such potentiation could not be obtained using diet restriction alone.

Busulfan therapy of central nervous system xenografts in athymic mice

We evaluated the antitumor activity of busulfan against a panel of tumor cell lines and xenografts in athymic nude mice derived from childhood high-grade glioma, adult high-grade glioma, ependymoma, and medulloblastoma. Busulfan displayed similar activity against a panel of four medulloblastoma cell lines (D283 Med, Daoy, D341 Med, and D425 Med) and four corresponding sublines with laboratory-generated or clinically acquired resistance to 4-hydroperoxycyclophosphamide [D283 Med (4-HCR), Daoy (4-HCR), D341 Med (4-HCR), and D458 Med] and cross-resistance to melphalan. This is consistent with a nearly total lack of cross-resistance of busulfan to 4-hydroperoxycyclophosphamide. Busulfan was active in the therapy of all but one of the subcutaneous xenografts tested, with growth delays ranging from 14.3 days in D612 EP to 58.4 days in D528 EP. Busulfan produced statistically significant increases in the median survival of mice bearing intracranial D456 MG (66%-90%), D612 EP (18%-33%), and D528 EP (89%) xenografts. These studies suggest that busulfan may be active against medulloblastomas, high-grade gliomas, and ependymomas as well as against cyclophosphamide-resistant neoplasms.

Development of a model of melphalan-induced gastrointestinal toxicity in mice

The tolerated dose of melphalan is limited by bone marrow suppression; when this complication is ameliorated by bone marrow transplantation, the dose-limiting toxicity becomes gastrointestinal mucositis. No intervention to date has been successful in modulating this life-threatening complication of melphalan. We conducted studies to develop a murine model of melphalan-induced gastrointestinal toxicity to facilitate the preclinical identification of effective strategies for reducing this toxicity. Melphalan given at the 90% lethal dosage produced severe gastrointestinal mucositis and mortality (13 of 23 treated mice). Syngeneic bone marrow transplantation, effective in preventing the myeloablation produced by total-body irradiation, was ineffective in preventing melphalan-induced mortality (16 of 23 treated mice), indicating that gastrointestinal mucositis was the dose-limiting toxicity. On the basis of the results of previous studies, which revealed that depletion of glutathione enhances the antineoplastic activity of melphalan and that glutathione is required for murine intestinal function, we attempted to modulate melphalan-induced gastrointestinal toxicity by the administration of glutathione (8-10 mmol/kg given in 1 ml sterile water by gavage at 12-h intervals for 4-8 doses). Glutathione therapy failed to produce a significant increase in mucosal glutathione content in animals treated with melphalan plus glutathione gavage as compared with those receiving melphalan alone (P > 0.05), and histologic mucosal injury secondary to melphalan was not reduced. The administration of glutathione in the presence or absence of concomitant bone marrow transplantation did not decrease melphalan-induced mortality (melphalan alone, 16/26 deaths; melphalan plus glutathione, 14/25 deaths; melphalan plus glutathione plus bone marrow transplantation, 20/26 deaths). Studies using a reduced melphalan dose (50% lethal dosage) produced similar results, with no survival benefit being seen following glutathione administration. Our studies suggest that melphalan-induced mucositis can be studied in a mouse model in which this complication is dose-limiting. Although glutathione administration at the dose and schedules initially studied is not effective in reducing this damage, other therapeutic strategies such as the use of alternative glutathione regimens or other thiols can be effectively studied in this system.

Acyclovir: discovery, mechanism of action, and selectivity

The reasons for acyclovir's activity and selectivity in cells infected with HSV or VZV may be summarized as follows: 1. Activation by a HSV- or VZV-specified TK. 2. Greater sensitivity of viral DNA polymerase than of the cellular polymerases to ACV-TP. 3. Inactivation of the viral DNA polymerase, but not the cellular polymerases, by ACV-TP. 4. Chain termination of viral DNA by incorporation of ACV-MP. For the Epstein-Barr virus, which is also sensitive to acyclovir, there is no selective activation in infected cells [Colby et al., 1981], but the viral polymerase can be inhibited by very low levels of ACV-TP [Datta et al., 1980]. For HCMV, the activation of acyclovir is very poor but the viral polymerase is also more sensitive to ACV-TP than the cellular polymerases. One of the important contributions of acyclovir was the demonstration for the first time that a compound could prevent the DNA replication of a DNA virus at concentrations far below those that affect cellular DNA synthesis. As we all know, in the past 15 years there has been a complete rejuvenation of antiviral chemotherapy. I think it is very fortunate that we changed our outlook on the possibility of making potent and selective antiviral agents in time so that, when the AIDS epidemic came along, we did not feel completely at a loss on ways to attack viral disease.

Effects of glutathione or polyamine depletion on in vivo thermosensitization

Investigations with the melphalan-sensitive and -resistant human rhabdomyosarcoma xenografts TE-671 and TE-671 MR were performed to examine the effect of glutathione and polyamine modulation on thermosensitivity. Regimens of intraperitoneally injected and orally administered buthionine sulfoximine were utilized to achieve glutathione depletion to 8.7% and 13% of control levels in TE-671 and TE-671 MR, respectively. Animals treated with L-buthionine-S,R-sulfoximine and 42 degrees C or 43 degrees C hyperthermia for 70 min showed no detectable growth delays beyond those observed for hyperthermia alone. Hyperthermia at 42 degrees C of disaggregated TE-671 and TE-671 MR xenografts following growth in short-term culture was performed following preincubation with buthionine sulfoximine or 0.9% saline. Buthionine sulfoximine-mediated glutathione depletion produced a significant increase in hyperthermia-induced cytotoxicity only with TE-671 MR at 43 degrees C. Polyamine depletion was achieved with a 7-day orally administered course of MDL 72.175DA [(2R,5R)-6-heptyne,5-diamine dihydrochloride], an irreversible inhibitor of ornithine decarboxylase. Although this treatment caused significant depletion of intracellular putrescine and spermidine levels, spermine levels remained relatively unaffected. No significant growth delays were observed in either xenograft line for animals treated with MDL 72.175DA or MDL 72.175DA plus hyperthermia as compared with untreated controls. These results contrast with previous work performed in vitro showing synergism between glutathione or polyamine depletion and hyperthermia, and indicate that further studies are needed.

Hyperthermia-induced enhancement of melphalan activity against a melphalan-resistant human rhabdomyosarcoma xenograft

The effects of regional hyperthermia (42 degrees C for 70 min) on the antitumor activity of melphalan were examined in athymic mice bearing melphalan-resistant human rhabdomyosarcoma (TE-671 MR) xenografts growing in the right hind limb, and results were compared with similar studies of melphalan-sensitive (TE-671) parent xenografts. Melphalan alone at a dose of 36 mg/m2 (0.5 of the 10% lethal dose) produced growth delays of 4.1 to 10.2 days in TE-671 MR xenografts and 21.8 to 28.7 days in TE-671, respectively. Hyperthermia alone produced growth delays of 0.9 days in TE-671 MR xenografts and 0.8 days in TE-671. Combination therapy with melphalan and hyperthermia produced growth delays of 7.2 to 13.3 days in TE-671 MR xenografts and 34.3 to 42.8 days in TE-671, respectively, representing a mean thermal enhancement ratio of 1.7 in TE-671 MR and 1.5 in TE-671. Measurement of glutathione levels in TE-671 MR xenografts following treatment with melphalan, hyperthermia, or melphalan plus hyperthermia revealed significant reductions in glutathione content with the nadir (60% of control values) seen 6 h following treatment. Glutathione levels in TE-671 xenografts following identical therapy revealed no differences from control values. Hyperthermia plus melphalan did not result in a higher tumor-to-plasma melphalan ratio compared with treatment with melphalan alone in either TE-671 MR or TE-671 xenografts. These studies suggest that heat-induced alterations in tumor glutathione or melphalan levels are not responsible for the increase in melphalan activity produced by hyperthermia. Combination therapy with melphalan plus regional hyperthermia offers promise for treatment of melphalan-resistant neoplasms.

Enhancement of melphalan-induced gastrointestinal toxicity in mice treated with regional hyperthermia and BSO-mediated glutathione depletion

Both hyperthermia and glutathione depletion have been shown to increase the antineoplastic activity of melphalan. Investigations were carried out to define the toxicity and activity of melphalan given in conjunction with local (right hind limb) hyperthermia and L-buthionine-SR-sulphoximine (BSO)-mediated glutathione depletion to athymic mice bearing the melphalan-resistant human rhabdomyosarcoma xenograft TE-671 MR. Administration of 0.5 of the 10% lethal dose of melphalan to mice treated with BSO and hyperthermia (42 degrees C for 70 min) resulted in a 53% mortality rate. The mortality rates for mice treated with melphalan alone (2.5%), hyperthermia alone (0%), melphalan plus BSO (13.5%), melphalan plus hyperthermia (12.0%) and BSO plus hyperthermia (0%) were substantially lower than triple therapy. Histological examination of kidney, liver, colon, and small intestine sections taken from non-tumour-bearing animals revealed a marked increase in damage to the small intestine (cryptal necrosis and epithelial denudement) in animals receiving triple therapy compared with animals receiving any other treatment combination. Gavage administration of sterile water (1 ml twice a day) completely prevented mortality in animals receiving triple therapy. Treatment of tumour-bearing animals with triple therapy plus gavage demonstrated a statistically significant increase in tumour growth delay compared with animals receiving any other treatment combination.

Pharmacologic purging of malignant T cells from human bone marrow using 9-beta-D-arabinofuranosylguanine

Arabinosylguanine (araG) is a nucleoside analog that is rapidly converted by cells of the T lymphoid lineage to its corresponding arabinosylguanine nucleotide triphosphate, resulting in inhibition of DNA synthesis and selective in vitro toxicity to T lymphoblastoid cell lines as well as to freshly isolated leukemia cells from patients with T cell acute lymphoblastic leukemia. In this report, we demonstrate that araG is an effective agent to use for chemoseparation of malignant T lymphoblasts from human bone marrow. When freshly isolated human T leukemia cells or T lymphoblastoid cells were treated with 100 microM araG for 18 hr, up to 6 logs of clonogenic T cells could be eliminated without appreciable toxicity to the normal myeloid, erythroid, and megakaryocytoid clonal progenitor cells. We discuss the use of this agent in ex vivo elimination of residual malignant T cells from marrow of patients requiring myeloablative chemotherapy with autologous bone marrow rescue.

Therapeutic analysis of melphalan-resistant human rhabdomyosarcoma xenograft TE-671 MR

Investigations with the melphalan-resistant human rhabdomyosarcoma xenograft TE-671 MR were carried out to identify patterns of cross-resistance and collateral sensitivity and to define the mechanism(s) mediating melphalan resistance. TE-671 MR was cross-resistant to thio-TEPA, mitomycin, vincristine, and cisplatin, and partially resistant to chlorambucil and cyclophosphamide. TE-671 MR and the parent line TE-671 were both resistant to 1,3-bis(2-chloroethyl)-nitrosourea and expressed similar levels of O6-alkylguanine-DNA alkyltransferase. TE-671 MR retained full sensitivity to actinomycin D and demonstrated enhanced sensitivity to VP-16 compared to TE-671. Treatment of TE-671 MR with melphalan plus VP-16 resulted in greater than additive growth delays. The frequency of hypoxic regions was similar in TE-671 MR and TE-671, respectively. Measurement of tumor-to-plasma levels at 180 min following i.p. administration of melphalan at 0.5 of the 10% lethal dosage showed mean tumor-to-plasma ratios of 3.81 in TE-671 MR and 7.38 in TE-671, respectively. The lower drug levels in TE-671 MR may be contributing to the resistance to melphalan and thus indicate the need for further studies to define the reasons for these differences in tumor drug level.

Positive therapeutic interaction between thiopurines and alkylating drugs in human glioma xenografts

We used human anaplastic glioma xenografts to evaluate the therapeutic efficacy of combinations of alkylating drugs, either 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), 1-(2-chloroethyl)-3-(2,5-dioxo-3-piperidyl)-1-nitrosourea (PCNU), or procarbazine, and thiopurines, either 6-mercaptopurine (6MP) or 6-thioguanine (6TG). Using growth delay as the endpoint in subcutaneous (s.c.) tumors and increased life span as the endpoint in intracranial (i.c.) tumors, we found that combinations of chloroethylnitrosoureas (CENUs) and thiopurines were significantly more active than either type of agent alone. In contrast, combinations of procarbazine and thiopurines were not significantly more active than procarbazine alone. The therapeutic potentiation of the CENU was greater when the latter was given on the 4th day of the thiopurine treatment cycle than when it was given on the 1st day. Characterization of the interaction between CENUs and thiopurines also revealed a supraadditive therapeutic response at higher BCNU doses in combination with 6TG. Interaction between the nitrosoureas and the thiopurines probably occurs in the guanine base of tumor DNA and has important therapeutic implications.

Synergistic interactions between cyclophosphamide or melphalan and VP-16 in a human rhabdomyosarcoma xenograft

Based on previous work demonstrating the activity of cyclophosphamide and melphalan in a series of human medulloblastoma and rhabdomyosarcoma cell lines and transplantable xenografts, investigations were conducted to define the effects of combining cyclophosphamide or melphalan with VP-16. These studies demonstrated a synergistic interaction between cyclophosphamide and VP-16 and melphalan and VP-16 in the treatment of the human rhabdomyosarcoma cell line TE-671 growing in athymic mice. The combination of cyclophosphamide or melphalan with VP-16 may warrant consideration as a therapeutic strategy for solid tumors sensitive to bifunctional alkylating agents.

Establishment of a melphalan-resistant rhabdomyosarcoma xenograft with cross-resistance to vincristine and enhanced sensitivity following buthionine sulfoximine-mediated glutathione depletion

A melphalan-resistant human rhabdomyosarcoma xenograft, TE-671 MR, was established in athymic mice by serial melphalan treatment of the parent xenograft, TE-671, at the 10% lethal dosage (LD10); significant resistance was evident after ten passages of the tumor. TE-671 MR demonstrated a doubling time of 3.5 days and a latency period to 1000-mm3 tumors of 27.5 days. The glutathione level of TE-671 MR was 2.36 mumol/g tumor, wet weight, 2-fold higher than the parent line. The glutathione S-transferase activity of TE-671 MR was 117.8 mumol/min/mg protein, essentially unchanged from the parent line. Although TE-671 MR demonstrated cross-resistance to vincristine, dot blot analysis did not reveal an elevated expression of mdr1 mRNA in the resistant line. TE-671 MR demonstrated a 9.7-day growth delay following treatment with melphalan at the LD10 (compared to 20.9 days for the parent line). Treatment with L-buthionine-SR-sulfoximine (BSO) resulted in increased sensitivity to melphalan subsequently administered at 50% of the LD10 (melphalan alone, growth delays of 3.7 and 4.6 days in duplicate trials; melphalan plus BSO, growth delays of 7.2 and 9.8 days). Sensitivity to melphalan equal to that of the parent line TE-671 was not achieved, however. Treatment with BSO did not result in significantly enhanced sensitivity to subsequently administered vincristine (50% of the LD10) (vincristine alone, growth delays of 6.8 and 6.9 days in duplicate trials; vincristine plus BSO, growth delays of 10.9 and 7.5 days). These results suggest that generation of melphalan resistance may be associated with development of cross-resistance to vincristine; this resistance may be associated with (although not necessarily mediated by) glutathione elevation; this resistance may be partially overcome by BSO-mediated depletion of glutathione.

Nobel Lecture. The purine path to chemotherapy

Antimetabolites of purine metabolism have found a use as anti-leukaemic, antiprotozoal and antiviral drugs, in immunosuppression and transplantation, and in gout and hyperuricemia. Their mechanisms of action are reviewed.

The purine path to chemotherapy

Research on antimetabolites of nucleic acid purines led to drugs for the treatment of acute leukemia (6-mercaptopurine and thioguanine), gout and hyperuricemia (allopurinol), and herpesvirus infections (acyclovir), and for the prevention of organ transplant rejection (azathioprine).

Increased melphalan activity in intracranial human medulloblastoma and glioma xenografts following buthionine sulfoximine-mediated glutathione depletion

In previous studies we demonstrated that administration of buthionine sulfoximine (BSO) to athymic BALB/c mice bearing intracranial human glioma xenografts resulted in highly selective depletion of glutathione in neoplastic tissue with minimal effects on contralateral normal brain tissue. In the present study we treated athymic BALB/c mice bearing intracranial human glioma (D-54 MG) or medulloblastoma (TE-671) xenografts with melphalan alone or BSO followed by melphalan. Administration of BSO depleted intracellular glutathione to 7.5% of the control level. BSO plus melphalan resulted in a significant increase in median survival over that produced by melphalan alone: 45.3% versus 26.4% in TE-671 and 69% versus 27.6% in D-54 MG. These studies justify further efforts to modulate chemotherapeutic and radiotherapeutic interventions of primary malignant brain tumors by depletion of glutathione.

Buthionine sulfoximine-mediated depletion of glutathione in intracranial human glioma-derived xenografts

D-54 MG, a human glioma-derived continuous cell line growing as subcutaneous or intracranial xenografts in athymic mice, was found to be sensitive to the effects of D,L-buthionine-(SR)-sulfoximine, a selective inhibitor of gamma-glutamylcysteine synthetase. Intraperitoneal administration of one dose of buthionine sulfoximine (BSO, 5 mmol/kg) resulted in depletion of total intracellular glutathione to 57 and 47% of control 12 hr, and 73 and 23% of control 24 hr, after BSO in subcutaneous and intracranial xenografts respectively. Concurrent measurement of total glutathione in the contralateral (non-tumor-containing) cerebral hemisphere in mice bearing intracranial D-54 xenografts demonstrated insignificant depletion of glutathione. Multiple doses of BSO, at 12-hr intervals, resulted in further depletion to 27% (s.c.) and 16.5% (i.c.) of control 12 hr following the final dose of BSO. Quantitative analysis of BSO delivery to xenograft and contralateral brain tissue revealed transfer constants, K1, of 15.8-24.1 x 10(-3) and 2.4 x 10(-3) ml.g-1.min-1 for xenograft and "normal" brain respectively. This highly selective depletion of glutathione in neoplastic tissue versus surrounding non-neoplastic host tissue may have therapeutic implications for the rational use of chemotherapeutic and radiotherapeutic intervention.

Melphalan transport, glutathione levels, and glutathione-S-transferase activity in human medulloblastoma

Melphalan transport, glutathione levels, and glutathione-S-transferase activity were measured in two continuous human medulloblastoma cell lines and transplantable xenografts in athymic nude mice, TE-671 and Daoy. In vitro mean glutathione levels were 10.06 nmol/10(6) cells in TE-671 and 2.96 nmol/10(6) cells in Daoy. In vitro mean glutathione-S-transferase values were 91.52 nmol/min/mg protein in TE-671 and 50.31 nmol/min/mg protein in Daoy. Transport studies revealed kinetic parameters of Km = 108.3 microM, Vmax = 363.1 pmol/10(6) cells/min in TE-671 and Km = 111.7 microM, Vmax = 180.6 pmol/10(6) cells/min in Daoy. Melphalan transport was inhibited by both DL-alpha-2-aminobicyclo[2.2.1]heptane-2- carboxylic acid and sodium ion depletion in TE-671 and Daoy cells in vitro, indicating that both systems of amino acid transport are functional in these medulloblastoma lines. In vivo s.c. xenograft glutathione values were lower (7.79 nmol/mg protein) in TE-671 than in Daoy (13.68 nmol/mg protein). The mean plasma concentration in mice given a 10% lethal dose (71.3 mg/m2) of melphalan i.p. was 50.3 microM at 10 min, with the half-life of 29.9 min. At this dose, s.c. xenograft levels were 2- to 3-fold higher in TE-671 than in Daoy tumors for the 3-h period measured. These studies demonstrate transport parameters confirming facilitated transport of melphalan in human medulloblastoma, a mean murine plasma melphalan concentration (following treatment with melphalan) above the in vitro drug dose at which there is a 90% reduction in the number of colonies in comparison to controls for TE-671 and Daoy for 2 h, and glutathione and glutathione-S-transferase levels in the same range previously reported in other melphalan-sensitive and melphalan-resistant human tumors. Future work with spontaneous and acquired melphalan-resistant human medulloblastoma cell lines and xenografts will define the role of these mechanisms in mediating drug resistance.

Experimental chemotherapy of human medulloblastoma cell lines and transplantable xenografts with bifunctional alkylating agents

A series of bifunctional alkylators were tested against the genotypically and phenotypically heterogeneous continuous human medulloblastoma cell lines, TE-671, Daoy, and D283 Med in vitro and against TE-671 and Daoy growing as s.c. and intracranial xenografts in athymic mice. Drugs tested included melphalan, cyclophosphamide, iphosphamide, phenylketocyclophosphamide, thiotepa, 1,3-bis(2-chloroethyl)-1-nitrosourea (in vivo), and busulfan (in vivo). Melphalan and phenylketocyclophosphamide were the most active agents in vitro with drug doses at which there is a 90% reduction in the number of colonies in comparison to controls of 2.13, 5.29, and 4.72 microM for melphalan and 4.60, 5.01, and 4.34 microM for phenylketocyclophosphamide against TE-671, D283 Med, and Daoy, respectively. Melphalan, cyclophosphamide, iphosphamide, phenylketocyclophosphamide, and thiotepa produced significant growth delays against s.c. TE-671 and Daoy xenografts, while no activity could be demonstrated for 1,3-bis(2-chloroethyl)-1-nitrosourea or busulfan. Melphalan, cyclophosphamide, iphosphamide, and thiotepa also produced significant increases in median survival in mice bearing intracranial TE-671 and Daoy xenografts. These results extend our previous studies demonstrating the antitumor activity of nitrogen and phosphoramide mustard-based bifunctional alkylating agents in the treatment of human medulloblastoma continuous cell lines and transplantable xenografts, and support the continued use of these agents in clinical trials.

Enhanced melphalan cytotoxicity following buthionine sulfoximine-mediated glutathione depletion in a human medulloblastoma xenograft in athymic mice

The effect and therapeutic consequences of buthionine-(SR)-sulfoximine (BSO)-mediated depletion of glutathione in the human medulloblastoma-derived cell line, TE-671, growing as s.c. xenografts in athymic nude mice were examined. The glutathione content of the s.c. xenografts was 1.11 +/- 0.15 mumol/g (7.79 +/- 1.61 nmol/mg of protein). Administration i.p. to tumor-bearing mice of D,L-BSO (two doses at 12-h intervals; 5 mmol/kg) depleted the glutathione content of the xenografts to 25.7% of control. Administration of a 30 mM solution of L-BSO in drinking water for 96 h depleted the glutathione content to 17.4% of control. Depletion of glutathione with these regimens resulted in a significant increase in the s.c. tumor growth delay over that produced by melphalan alone: 17.2 days versus 12.6 days for D,L-BSO (i.p.) plus melphalan versus melphalan and 22.9 days versus 16.6 days for L-BSO (p.o.) plus melphalan versus melphalan. These studies demonstrate the increased cytotoxicity of melphalan resulting from BSO-mediated depletion of glutathione in human medulloblastoma and support further efforts to modulate the chemosensitivity and radiosensitivity of this tumor by modulation of glutathione.

Influence of glutamine on the growth of human glioma and medulloblastoma in culture

Cellular supply of glutamine, an essential substrate for growth, is derived from extracellular fluid and de novo synthesis. We investigated the relative importance of these sources to the growth of six human anaplastic glioma- and one human medulloblastoma-derived permanent cell lines. Exogenous glutamine was limiting for the proliferation of glioma-derived lines D-54 MG, U-118 MG, and U-251 MG. In contrast, medulloblastoma-derived line TE-671 and glioma-derived lines U-373 MG, D-245 MG, and D-259 MG grew in the absence of supplemental glutamine. Two cell lines with contrasting glutamine requirements, D-54 MG and TE-671, were used to explore the pharmacological interference with glutamine metabolism. DL-alpha-Aminoadipic acid, a reported glutamic acid analogue with gliotoxic properties, significantly inhibited the growth of both lines. These effects were reversed by increasing glutamine, suggesting that the major action of DL-alpha-aminoadipic acid is as a glutamine antagonist. In contrast, the glutamine synthetase inhibitor delta-hydroxylysine demonstrated activity only against TE-671. Acivicin and 6-diazo-5-oxo-L-norleucine, glutamine analogues available for clinical use, reduced the proliferation of both cell lines at pharmacological concentrations. Methionine sulfoximine, a glutamine synthetase inhibitor previously used clinically, produced marked growth inhibition only against TE-671. These findings indicate that the synthesis and utilization of glutamine are potentially exploitable targets for the chemotherapy of some human gliomas and medulloblastomas.

Combination chemotherapy in vitro exploiting glutamine metabolism of human glioma and medulloblastoma

The human glioma-derived cell line D-54 MG and the human medulloblastoma-derived cell line TE-671 have been shown to be sensitive in culture to the pharmacological interference with glutamine metabolism by acivicin, 6-diazo-5-oxo-L-norleucine, and methionine sulfoximine. Using as a guide the multiple contributions of glutamine to the biosynthesis of proteins, purines, and pyrimidines, we now have identified six additional antimetabolites active against these lines in vitro at clinically relevant concentrations. The 50% growth-inhibitory levels of the drugs against D-54 MG in 6-day continuous exposure experiments were: L-asparaginase, 0.057 IU/ml; 5-fluorouracil, 0.5 micrograms/ml; 6-mercaptopurine, 0.8 micrograms/ml; actinomycin D, 0.0007 micrograms/ml; N-phosphonacetyl-L-aspartic acid, 2.3 micrograms/ml; and 5-azacytidine, 0.2 micrograms/ml (3-day exposure. The corresponding 50% growth-inhibitory values in TE-671 were: L-asparaginase, 0.54 IU/ml; 5-fluorouracil, 1.5 micrograms/ml; 6-mercaptopurine, 4.7 micrograms/ml; actinomycin D, 0.00044 micrograms/ml; N-phosphonacetyl-L-aspartic acid, 4.5 micrograms/ml; and 5-azacytidine, 0.49 micrograms/ml. Dipyridamole up to 10 micrograms/ml was inactive against both lines. The isobologram method was used to evaluate the effectiveness of several two-drug combinations which were biochemically designed. The sums of the optimal fractional inhibitory concentrations for the pairs were: acivicin plus L-asparaginase, 0.14; acivicin plus methionine sulfoximine, 0.40; 6-diazo-5-oxo-L-norleucine plus methionine sulfoximine, 0.60; acivicin plus 6-mercaptopurine, 1.0, all in TE-671; and acivicin plus 5-fluorouracil, 0.79, in D-54 MG. Our findings suggest that an antimetabolite regimen exploiting glutamine sensitivity might improve the chemotherapy of some human gliomas and medulloblastomas.

An overview of the role of nucleosides in chemotherapy

A wide variety of nucleosides have been synthesized which interfere with the functions of natural nucleosides at many different loci. These analogues may act as substrates or inhibitors of nucleoside-metabolizing enzymes or may bind to cell membrane receptors. Because of quantitative and qualitative differences in the enzymes and receptors of various tissues and species, the nucleoside analogues have found important uses as antitumor, antiviral, antiparasitic and immunomodulating agents.

Identification of small DNA fragments synthesized in herpes simplex virus-infected cells in the presence of acyclovir

The effect of acyclovir on DNA synthesized in cells infected with herpes simplex virus type 1 was examined. DNA that was synthesized in infected cells in the presence of acyclovir during a short pulse with [3H]thymidine remained near the top of an alkaline sucrose gradient after centrifugation. The sedimentation characteristics of labeled DNA were not changed after chasing in isotope-free medium. The slowly sedimenting DNA was identified as viral in origin by hybridization to purified herpes simplex virus nucleocapsid DNA. When cells were infected with acyclovir-resistant virus containing mutations in the polymerase gene, the viral DNA synthesized in the presence of acyclovir was chased into high-molecular-weight DNA. These findings are consistent with chain termination of herpes simplex virus DNA in virus-infected cells.

Metabolic studies of high doses of allopurinol in humans

In animals and in humans given high doses of allopurinol, the oxidation of allopurinol to oxipurinol is inhibited, resulting in a higher proportion of unchanged allopurinol and of allopurinol riboside in plasma and urine than is seen at low doses. The dose which produces this inhibition of allopurinol oxidation is higher in rodents than in man or in the dog. Urinary orotate and orotidine increased in proportion to the dose of allopurinol. These increased levels of orotate would be expected to compete more effectively with 5-fluorouracil for conversion to a nucleotide by orotate phosphoribosyltransferase. Since allopurinol and allopurinol riboside are active against leishmaniae in vitro, it may be possible to attain therapeutic levels of allopurinol and allopurinol riboside in vivo by using high doses of allopurinol.

Disposition in the dog and the rat of 2, 6-diamino-9-(2-hydroxyethoxymethyl)purine (A134U), a potential prodrug of acyclovir

2,6-Diamino-9-(2-hydroxyethoxymethyl)purine (A134U), the 6-deoxy-6-amino analog of the antiviral agent acyclovir (ACV), was administered orally to dogs and rats. Plasma concentration-time profiles and urinary excretion of A134U and its deamination product, ACV, were determined. Mean peak plasma ACV concentrations achieved in the dog were 57, 156 and 285 microM after A134U doses of 20, 50 and 120 mg/kg, respectively, and increased in near proportion to the dose. The urinary recovery of ACV accounted for 60-92% of the two lower doses, but only 40-58% of the highest dose. In the rat, peak plasma ACV concentrations were 3.1 and 10.7 microM, respectively, after 20- and 50-mg/kg doses of A134U. After 5- and 20-mg/kg oral doses of [2-14C]A134U, the urinary recovery of ACV (20-27%) accounted for 59 to 76% of the absorbed dose. The remainder was excreted largely as unchanged A134U, with negligible (0.4-1.3%) biotransformation to inactive metabolites. Except for small decreases in absorption and increases in deamination, no change in the metabolism of A134U was observed after its repeated oral administration to rats. Oral dosing of dogs and rats with A134U resulted in peak plasma concentrations and total urinary recoveries of ACV greater than those observed after equivalent oral doses of ACV, suggesting that A134U might be an effective prodrug of ACV for use in the oral therapy of herpes simplex virus infections.

Preclinical toxicology studies with acyclovir: ophthalmic and cutaneous tests

Topical formulations of acyclovir (ACV) were tested in animals to define potential for tissue irritation and systemic toxicity. Acyclovir ointments (5 and 10% concentrations in polyethylene glycol vehicle) produced no sign of dermal irritation or systemic toxicity when applied to shaved abraded and intact skin of guinea pigs for 24 consecutive days. Solutions (0.9% normal saline vehicle) of ACV did not sensitize guinea pigs when 10 sensitizing doses and a challenge dose were injected intradermally. Petrolatum base ophthalmic ointments containing 1 and 3% ACV did not produce significant ocular irritation when applied to the corneas of New Zealand White rabbits 5 times each day for 21 consecutive days. A 6% petrolatum base ointment produced mild conjunctival irritation but no sign of corneal or iridic toxicity. Mean concentrations of 2.53 microM ACV were found in aqueous humor 2 hours after a 1 cm ribbon (21 mg) of 3% ophthalmic ointment was placed in the eyes of rabbits. A single treatment with a topical ointment containing 5% ACV in polyethylene glycol vehicle produced minimal irritation when placed in the eyes of New Zealand White rabbits.

Preclinical toxicology studies with acyclovir: acute and subchronic tests

Acyclovir (ACV), a new antiherpes drug, was evaluated for toxicity in a series of acute and subchronic toxicity tests. Oral LD50 values were greater than 10 000 mg/kg in male ICR mice and greater than 20 000 mg/kg in male Long Evans rats. When ACV was given iv, the LD50 was 405 mg/kg for male mice and greater than 600 mg/kg for male rats. Additionally, LD50 values for male rats treated sc were 1070, 790, 678, and 650 mg/kg in rats that were respectively, 3, 10, 28 and 71 days old indicating that very young rats were not more sensitive to acute toxic effects of ACV. There were no signs of toxicosis in CD-1 mice given ACV by gavage at dose levels of 50, 150 and 450 mg/kg/day for 1 month. Obstructive nephropathy occurred in rats given 20, 40 and 80 mg/kg/day once each day by rapid iv injection for 3 weeks. Both 5 and 10 mg/kg/day were no effect dose levels. Renal damage caused by precipitation of drug crystals in renal tubules and collecting ducts in rats given ACV by rapid iv injection was readily reversible within 2 weeks. Beagle dogs were given doses of 10, 20, 25, 50 and 100 mg/kg b.i.d. by rapid iv injection for 1 month. All 8 dogs given 100 mg/kg b.i.d. died by the 8th day of treatment; 5 of 8 dogs given 50 mg/kg b.i.d. died after 21 to 31 days of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Preclinical toxicology studies with acyclovir: carcinogenicity bioassays and chronic toxicity tests

Acyclovir (ACV), a nucleoside analog that is a new herpes-specific antiviral drug, was given by gavage at 50, 150 and 450 mg/kg/day to Sprague Dawley rats and Swiss mice for most of their lifetime to assess chronic toxicity and carcinogenicity. Treatment with ACV did not shorten the lifespan of either rats or mice. In fact, female mice given 150 and 450 mg/kg/day had significantly longer mean durations of survival than control female mice when analyzed by the life table technique. There were no signs of toxicosis produced by chronic exposure to ACV in either the rats or mice, and there was no drug-related increase in neoplasms in either species. Four groups of Beagle dogs were initially given daily oral doses of 15, 45 or 150 mg/kg ACV in a 1 year chronic toxicity study. Dogs treated at 150 mg/kg/day vomited, had diarrhea, consumed less feed and lost weight within 2 weeks. Dogs treated at 45 mg/kg/day also had minimal signs of gastrointestinal toxicosis. These dose levels were then decreased to 60 and 30 mg/kg/day for the rest of the one year test period. With the exception of occasional and inconsistent emesis and diarrhea, the 60 mg/kg/day dose level was well tolerated. Some mid and high dose dogs had sore paws due to erosion of footpads and cracking, splitting and loosening of the nails first becoming evident during the 13th week of the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of acyclovir and its metabolites on hypoxanthine-guanine phosphoribosyltransferase

Acyclovir [9-(2-hydroxyethoxymethyl)guanine], a clinically useful anti-herpesvirus agent, was a weak inhibitor (Ki = 190 microM) of hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) from human erythrocytes. Nevertheless, this acyclic nucleoside analog was a more effective inhibitor than were its natural counterparts, guanosine (Ki = 1400 microM) and deoxyguanosine (Ki = 570 microM). The two oxidized metabolites of acyclovir, 9-carboxymethoxymethylguanine (Ki = 720 microM) and 8-hydroxy-9-(2-hydroxyethoxymethyl)guanine (Ki greater than 2000 microM), were less inhibitory than was the parent drug. None of the phosphorylated metabolites of acyclovir was as potent an inhibitor of HGPRTase as was GMP (Ki = 4 microM). However, the Ki value for acyclovir monophosphate was similar to that of dGMP (12 microM). The Ki values for acyclovir diphosphate (8.3 microM) and triphosphate (30 microM) were less than those for dGDP (110 microM) and dGTP (140 microM). The levels of these phosphate esters of acyclovir in cultured monkey kidney (Vero) and human embryo fibroblast (WI38) cells exposed to therapeutic levels of the drug were well below the observed Ki values. However, in herpesvirus-infected WI38 cells the levels of the phosphate esters of acyclovir were high enough potentially to inhibit the enzyme. Although inhibition of this enzyme by the phosphorylated metabolites of acyclovir may occur in these infected cells, concentrations of the drug very much higher than the EC50 concentration were required to achieve inhibitory levels. It is, therefore, unlikely that this inhibition contributes significantly to the antiviral activity.

Pyrazolo[3,4-d]pyrimidine ribonucleosides as anticoccidials. 3. Synthesis and activity of some nucleosides of 4-[(arylalkenyl)thio]pyrazolo[3,4-d]pyrimidines

Ribonucleosides of 4-(alkylthio)-1H-pyrazolo[3,4-d]pyrimidines have been shown to be useful anticoccidial agents [Krenitsky, T. A.; Rideout, J. L.; Koszalka, G. W.; Inmon, R. B.; Chao, E. Y.; Elion, G. B.; Latter, V. S.; Williams, R. B. J. Med. Chem. 1982, 25, 32. Rideout, J. L.; Krenitsky, T. A.; Elion, G. B. U.S. Patent 4299 283, 1981]. In that study, the unsaturated 4-allylthio and 4-crotylthio derivatives (19 and 20) were shown to be more active in vivo against Eimeria tenella than their saturated congeners; therefore, some unsaturated (arylalkyl)thio derivatives were synthesized and investigated as anticoccidial agents. The novel compounds in this study (2 to 18) were prepared by the alkylation of 4-mercapto-1-beta-D-ribofuranosyl-1H-pyrazolo[3,4-d]pyrimidine (1), which was prepared by an enzymatic method. The (E)-4-cinnamylthio derivative (2) and the 5'-monophosphate (18) were the most active compounds against E. tenella in vivo. None of the analogues with substituents in the aryl moiety (3 to 13) was more active than 2 in vivo. The geometry about the double bond was important, since the (Z)-4-cinnamylthio derivative (14) was inactive both in vitro and in vivo. The 4-(3-phenylpropynyl)thio and 4-(5-phenyl-2,4-pentadienyl)thio derivatives (15 and 16) were at least as active as 2 in vitro; however, they were less active than 2 in vivo. Compound 2 was effective in vivo against E. tenella, E. necatrix, E. maxima, and E. brunetti; these species of Eimeria were controlled when 2 was given in the diet at levels upt to 100 ppm. Infections in vivo due to E. acervulina were controlled by 2 only at about 800 ppm. The broad spectrum of anticoccidial activity shown by 2 represents a significant improvement over the activities reported for related compounds [Krenitsky, T. A.; Rideout, J. L.; Koszalka, G. W.; Inmon, R. B.; Chao, E. Y.; Elion, G. B.; Latter, V. S.; Williams, R. B. J. Med. Chem. 1982, 25, 32].

The biochemistry and mechanism of action of acyclovir

Acyclovir, 9-(2-hydroxyethoxymethyl)guanine, is an acyclic nucleoside analogue which has a high activity and selectivity for herpes viruses, particularly herpes simplex viruses types 1 and 2 and varicella zoster virus. This selectivity is due to the initial activation of the drug by phosphorylation by a herpes virus-specified thymidine kinase. Normal cellular enzymes do not phosphorylate acyclovir to any significant degree. Acyclovir monophosphate is subsequently converted to a triphosphate which is a more potent inhibitor of herpes virus DNA polymerases than of cellular DNA polymerases. The relationship between the amount of acyclovir triphosphate formed and its inhibition constant (Ki) for the particular viral or cellular DNA polymerase is predictive of the inhibitory activity of acyclovir on DNA replication.