O.sup.6 -Substituted Guanine Compounds and Methods for Depleting O.sup.6 -Alkylguanine-Dna Alkyltransferase Levels

1. Field of the Invention

The present invention relates to O.sup.6 -benzyl-substituted guanine compounds which exhibit the ability to deplete O.sup.6 -alkylguanine-DNA alkyltransferase (AGT) activity and to methods for the administration thereof to enhance chemotherapeutic treatment of tumor cells. More specifically, the present invention relates to O.sup.6 -benzylguanine and related O.sup.6 -benzylguanine derivatives wherein the benzyl substituent may be substituted at the ortho, meta or para position. The present invention also relates to methods for administering compositions containing these compounds to a host in order to reduce the activity of AGT in tumor cells of the host which increases host responsiveness to anti-neoplastic alkylating agents such as streptozotocin, procarbazine or dacarbazine as well as to chloroethylating agents, such as chloroethylnitrosoureas or chloroethyl triazenes, subsequently or simultaneously administered to the host.

2. Description of Related Art

Chemotherapeutic chloroethylating agents have some clinical utility against a number of neoplasms but in general have only limited effectiveness in killing tumor cells. The marginal usefulness of these agents may be explained on a cellular level by the local concentration of DNA repair proteins present in the tumor. There is strong evidence that the repair protein, O.sup.6 -alkylguanine-DNA alkyltransferase (AGT), present to some degree in all mammalian tumors studied to date, is responsible for protecting cells from the antitumor effects of chloroethylating agents such as chloroethylnitrosoureas. The AGT acts to remove the initial DNA adduct formed at the O.sup.6 -position of guanine by these agents before this adduct can be converted to a lethal lesion. Efforts to reduce the level of AGT in order to increase the sensitivity of cells to the chemotherapeutic effects of chloroethylating or other alkylating agents would be advantageous to therapy which involves cytotoxic lesions at the O.sup.6 -position of guanine. Furthermore, a specific decrease in AGT levels allows one to study the biological effects of alkylating agents.

Previous studies have indicated that the chemotherapeutic effectiveness of chloroethylating and methylating agents can be enhanced in human cells in culture by pretreatment with O.sup.6 -alkylguanines. Earlier investigations demonstrated that agents such as O.sup.6 -methyl-and O.sup.6 -n-butylguanine decreased AGT levels in human tumor cells in culture which led to an enhancement in the sensitivity of these cells to the cytotoxic effects of chloroethylating agents (Dolan et al, Cancer Research, 46, pp. 4500-4504, (1986)). Subsequent studies demonstrated that there was a dose dependent decrease in AGT levels in human colon xenograft tumors carried in nude mice after intraperitoneal injections of O.sup.6 -methylguanine (Dolan et al, Cancer Chemother. Pharmacol., 25, pp. 103-108, (1989)). However, these compounds were not potent enough to be likely to be useful for treatment of patients since the rate of AGT depletion was slow and the extent of depletion was not complete even with very high doses.

O.sup.6 -benzylguanine differs from O.sup.6 -methyl- and O.sup.6 -n-butylguanine in that the alkyl substituent is replaced with a benzyl group and it is a known intermediate compound. See Bowles et al, J. Med. Chem., 6, pp. 471-480 (1963); Frihart et al, J. Am. Chem. Soc., 95, pp. 7174-7175 (1973). However, the use of this compound in chemotherapeutic treatment has yet to be reported.

Therefore, it is an object of the present invention to provide novel compounds useful for effectively reducing AGT levels in tumor cells.

Another object of the present invention is to provide pharmaceutical compositions containing compounds which effectively reduce AGT levels in cells.

Yet a further object of the present invention is to provide methods for depleting AGT levels in tumor cells.

Still another object of the present invention is to provide methods for increasing host responsiveness to anti-neoplastic chloroethylating agents or other alkylating agents by administering to the host compositions containing compounds which deplete AGT levels in the tumor cells in the host.

The foregoing objects and others are accomplished in accordance with the present invention by providing compositions containing O.sup.6 -substituted guanine compounds of the following formula (I): ##STR2## wherein R is a benzyl group or a benzyl group substituted at the ortho, meta or para position with a substituent group such as halogen, nitro, aryl such as phenyl or substituted phenyl, alkyl of 1-4 carbon atoms, alkoxy of 1-4 carbon atoms, alkenyl of up to 4 carbon atoms, alkynyl of up to 4 carbon atoms, amino, monoalkylamino, dialkylamino, trifluoromethyl, hydroxy, hydroxymethyl, and SO.sub.n R.sub.1 wherein n is 0, 1, 2 or 3 and R.sub.1 is hydrogen, alkyl of 1-4 carbon atoms or aryl; and a pharmaceutically acceptable excipient. R is preferably a benzyl group or a benzyl group substituted with a substituent group such as halogen, nitro, alkyl of 1-4 carbon atoms, alkoxy of 1-4 carbon atoms, trifluoromethyl and hydroxymethyl. In an alternative embodiment, this composition may further include an anti-neoplastic alkylating agent, such as a chloroethylating agent.

In another embodiment of the present invention, a method is provided for depleting the activity of O.sup.6 -alkylguanine-DNA alkyltransferase (AGT) in tumor cells which comprises administering to a host compositions containing compounds of formula (I) and a pharmaceutically acceptable excipient. In a further embodiment of the present invention, a method is provided for increasing host responsiveness to anti-neoplastic chloroethylating or other alkylating agents by administering to the host compositions containing compounds of formula (I) and a pharmaceutically acceptable excipient and subsequently or simultaneously administering the anti-neoplastic chloroethylating or alkylating agents.

In yet a further embodiment of the present invention, novel compounds are provided having the chemical structure of formula (I) above, wherein R is a benzyl group substituted at the ortho, meta or para position with a substituent group such as halogen, nitro, aryl such as phenyl or substituted phenyl, alkyl of 1-4 carbon atoms, alkoxy of 1-4 carbon atoms, alkenyl of up to 4 carbon atoms, alkynyl of up to 4 carbon atoms, amino, monoalkylamino, dialkylamino, trifluoromethyl, hydroxy, hydroxymethyl, and SO.sub.n R.sub.1 wherein n is 0, 1, 2 or 3 and R.sub.1 is hydrogen, alkyl of 1-4 carbon atoms or aryl. R is preferably a benzyl group substituted with a substituent group such as halogen, nitro, alkyl of 1-4 carbon atoms, alkoxy of 1-4 carbon atoms, trifluoromethyl and hydroxymethyl.

Further scope of the applicability of the present invention will become apparent from the detailed description and drawings provided below. However, it should be understood that the detailed description and specific examples while indicating preferred embodiments of the invention are given by way of illustration only since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

The invention is further illustrated in the accompanying drawings wherein:

FIG. 1 is a graph showing the rate of inactivation of human alkyltransferase by O.sup.6 -benzylguanine and related compounds;

FIGS. 2(A) and 2(B) are graphs showing the loss of alkyltransferase activity in HT29 cells exposed to O.sup.6 -benzylguanine;

FIG. 3 is a graph showing the time course of the loss of alkyltransferase activity in HT29 cells;

FIGS. 4(A) and 4(B) are graphs showing the enhancement of cytotoxicity of chloroethylating agents by O.sup.6 -benzylguanine; and

FIG. 5 is a graph showing the depletion of alkyltransferase in mouse tissues by administration of O.sup.6 -benzylguanine.

The compounds encompassed by the present invention include compounds of the following formula (I): ##STR3## wherein R is a benzyl group or a benzyl group substituted at the ortho, meta or para position with a substituent group such as halogen, nitro, aryl such as phenyl or substituted phenyl, alkyl of 1-4 carbon atoms, alkoxy of 1-4 carbon atoms, alkenyl of up to 4 carbon atoms, alkynyl of up to 4 carbon atoms, amino, monoalkylamino, dialkylamino, trifluoromethyl, hydroxy, hydroxymethyl, SO.sub.n R.sub.1 where n is 0, 1, 2 or 3 and R.sub.1 is hydrogen, alkyl of 1-4 carbon atoms or aryl. R is preferably a benzyl group or a benzyl group substituted with a substituent group such as halogen, nitro, alkyl of 1-4 carbon atoms, alkoxy of 1-4 carbon atoms, trifluoromethyl and hydroxymethyl. Examples of such compounds include O.sup.6 -(p-chlorobenzyl)guanine and O.sup.6 -(p-methylbenzyl)guanine. All such O.sup.6 -benzylated guanines would be expected to exhibit AGT depletion characteristics and can be synthesized using appropriate adaptions known to those skilled in the art of the general description presented below.

O.sup.6 -Benzylguanine is a known intermediate compound which may be conventionally prepared as disclosed in Bowles et al, J. Med. Chem., 6, pp. 471-480 (1963) and Frihart et al, J. Am. Chem. Soc., 95, pp. 7144-7175 (1973). O.sup.6 -Benzylguanines substituted on the benzyl group at either the ortho, meta or para position may be synthesized by treating 2-amino-6-chloropurine with the alkoxide of benzyl alcohol bearing the desired ortho, meta or para substituent. For example, O.sup.6 -(p-chlorobenzyl)guanine and O.sup.6 -p-methylbenzyl)guanine may be prepared by treating 2-amino-6-chloropurine with an appropriate amount of sodium 4-chlorobenzyloxide in 4-chlorobenzyl alcohol or sodium 4-methylbenzyloxide in 4-methylbenzyl alcohol, respectively, followed by precipitation and purification of the product. Ortho and meta substituted products can be prepared in the analogous fashion.

The methods of the present invention include the administration of the above-noted compositions containing the compounds of formula (I) and a pharmaceutically acceptable excipient to a host.

In the methods according to the present invention, pharmaceutical compositions containing compounds according to the present invention are administered in an effective amount to a host so as to deplete AGT levels in tumor cells in the host thereby increasing host responsiveness to anti-neoplastic chloroethylating agents or other alkylating agents, so that subsequent or simultaneous administration of these anti-neoplastic agents to the host results in effective chemotherapeutic treatment for killing tumor cells.

The O.sup.6 -substituted guanine compound-containing compositions in accordance with the present invention may be used to reduce AGT levels in the treatment of various neoplasms, such as brain tumors, lymphomas, leukemias and solid tumors.

Analytical Methods. Proton nuclear magnetic resonance (.sup.1 H-NMR) spectra were recorded on a Varian XL 200 instrument interfaced to an Advanced data system. Samples were dissolved in dimethyl-de.sub.6 sulfoxide with tetramethylsilane as internal standard. Positive ion (+ve) fast atom bombardment (FAB) mass spectra (MS) were obtained with a reversed geometry VG Micromass ZAB-2F spectrometer interfaced to a VG 2035 data system. A mixture of dithiothreitol and dithioerythritol (1:1) was used as FAB matrix.

Chemicals and Drugs. Chemotherapeutic agents 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) (NSC 79037) and 2-chloroethyl(methylsulfonyl)methanesulfonate (Clomesone) (NSC 33847) were obtained from the Drug Synthesis and Chemistry Branch, Division of Cancer Treatment, National Cancer Institute, Bethesda, Md. [.sup.3 H]-Methylnitrosourea (5.6 Ci/mmol) was purchased from Amersham. All other chemicals were purchased from Fisher, Calbiochem, Sigma or Aldrich.

O.sup.6 -Benzylguanine, O.sup.6 - (p-chlorobenzyl) guanine and O.sup.6 -(p-methylbenzyl)guanine were prepared by treating 2-amino-6-chloropurine (0.018 tool) with 2.2 equivalents of either sodium benzyloxide, sodium 4-chlorobenzyloxide, or sodium 4-methylbenzyloxide in 30 g of either benzyl, 4chlorobenzyl or 4-methylbenzyl alcohol, respectively at 130.degree. C. for 6 h. The resulting suspensions were cooled to room temperature, treated with 2.5 ml glacial acetic acid with stirring and then poured into 1 liter of diethyl ether with vigorous stirring. The precipitated solids were collected by filtration and were crystallized from either H.sub.2 O or ethanol/H.sub.2 O (4:5) with decolorizing carbon as necessary to afford 50% yields of the respective O.sup.6 -benzylated guanine. O.sup.6 -(p-Chlorobenzyl)guanine; UV, .lambda..sub.max (pH7) 239, 283; .sup.1 H-NMR .delta. 5.48 (s,2,ArCH.sub.2), 6.29(br s,2,NH.sub.2, exchange with D.sub.2 O), 7.50(dd,4,Ar), 7.83(s,1,8-H), 12.43(br s,1,9 -H, exchanges with D.sub.2 O); +ve FAB MS, m/z 276 ([M+H].sup.+), 152([Guanine+H].sup.+), 125([C.sub.7 H.sub.6 Cl].sup.+). Anal. Clcd. for C.sub.12 H.sub.10 N.sub.5 OCl. 1/2H.sub.2 O: C, 50.62; H, 3.89; N, 24.60. Found: C,51.02; H,3.82; N,25.15. O.sup.6 -(p-Methylbenzyl)guanine: UV, .lambda..sub.max (pH 7) 241,283; .sup.1 H-NMR .delta. 2.31 (s,3,ArCH.sub.3), 5.43 (s,2,ArCH.sub.2), 6.28(br s,2,NH.sub.2) 7.30(dd,4,Ar), 7.80(s,1,8-H), 12.41(br s, 1,9-H, exchanges with D.sub.2 O); re FAB MS, m/z 256([M+H].sup.+), 152 ([Guanine+H].sup.+), 105([C.sub.8 H.sub.9 ].sup.+). Anal. Calcd. for C.sub.13 H.sub.13 N.sub.5 O: C,61.17; H,5.13; N,27.43. Found: C,61.05; H,5.13; N,27.14.

Cell Culture. The human colon carcinoma line HT29 was maintained in Dulbecco's modified Eagles medium containing 36 mM NaHCO.sub.3 supplemented with 10% fetal bovine serum, gentamycin (50 .mu.g/ml) and 3% glutamine in a humidified atmosphere of 5% CO.sub.2 at 37.degree. C. Cells were seeded weekly at 2.5.times.10.sup.5 cells/75 cm.sup.2 flask.

Three days prior to treatment, HT29 cells were lated at a density of 5.times.10.sup.6 cells/150 cm.sup.2 flask. After growth for 2-3 days, the medium on cells was replaced with fresh medium or medium containing O.sup.6 -alkylguanine derivatives. The treatment conditions such as concentration and time of exposure are indicated below in Examples 1-4. The cells were then harvested, collected by centrifugation at 1000.times.g in medium, washed with ice-cold phosphate buffered saline and the cell pellets stored at -80.degree. C. until assaying for alkyltransferase. Extracts were conventionally prepared as described previously in Domoradski et al, Carcinogenesis, 5, pp. 1641-1647 (1984).

Cell colony forming efficiency was determined by plating HT29 cells at a density of 1.times.10.sup.6 cells/25 cm.sup.2 flask 24 hours (h) prior to replacing medium with fresh medium or medium containing 10 .mu.M O.sup.6 -benzylguanine for 2 h. Cells were then treated with various concentrations of chloroethylating agent for 2 h at 37.degree. C. The medium was then replaced with the same medium as that used for the pretreatment for an additional 18 h, and the cells were replated at densities between 100 and 3200 per 25 cm.sup.2 flask. The cells were allowed to grow for 10-12 days and colonies were then washed with 0.9% saline, stained with 0.5% crystal violet in ethanol and counted.

The rate of inactivation of human alkyltransferase by O.sup.6 -benzylguanine and related compounds was measured and the results are shown in FIG. 1. The alkyltransferase from HT29 cells was incubated without addition () or with 2.5 .mu.M O.sup.6 -benzylguanine (.quadrature.), 0.5 .mu.M of O.sup.6 -benzylguanine (), 0.5 .mu.M of O.sup.6 -(p-chlorobenzyl)guanine (), or with 0.5 .mu.M of O.sup.6 -(p-methylbenzyl)guanine () in a total volume of 0.99 ml for the time shown and then the residual alkyltransferase activity was determined by addition of 0.01 ml containing the .sup.3 H-methylated DNA substrate for 30 min. The alkyltransferase activity was then determined and expressed as the percentage remaining based on that present in untreated controls (approximately 804 fmol/mg protein).

The loss of alkyltransferase activity in HT29 cells exposed to O.sup.6 -benzylguanine was measured and the results are shown in FIGS. 2(A) and 2(B). In the experiment shown in FIG. 2(A) the cells were exposed to O.sup.6 -benzylguanine or O.sup.6 -methylguanine at concentrations of 5 to 200 .mu.M as shown for 4 h. In the experiment shown in FIG. 2(B), the cells were exposed to O.sup.6 -benzylguanine (), O.sup.6 -(p-chlorobenzyl)guanine () O.sup.6 -(p-methylbenzyl)guanine () or 7-benzylguanine (.largecircle.) at concentrations of 0.01 to 2.5 .mu.M for 4 h.

A graph of a time course of the loss of alkyltransferase activity in HT29 cells is shown in FIG. 3. The cells were exposed to 100 .mu.M O.sup.6 -benzylguanine or O.sup.6 -methylguanine for the time indicated, then rinsed with PBS and extracts prepared for alkyltransferase analysis.

The enhancement of cytotoxicity of chloroethylating agents by O.sup.6 -benzylguanine was measured and the results are shown in FIGS. 4(A) and 4(B). Cultures of HT29 cells were exposed to fresh medium or medium containing 10 .mu.M O.sup.6 -benzylguanine for 2 h and then both sets were treated with the concentration shown of CCNU (FIG. 4(A)) or Clomesone (FIG. 4(B)) for 2 h. Colony forming efficiency determined as described under Materials and Methods is expressed as % survival (relative to zero drug addition).

Animal Treatment. Male CD-1 mice (approx. 37 g) were purchased from Charles River Breeding Laboratories, Inc. (Wilmington, Del.). Animals were treated with O.sup.6 -benzylguanine at a dose of 10.2 mg/kg body weight by intraperitoneal injection. The treatment consisted of injection of approximately 1 ml of a solution of 0.2 mg/ml of O.sup.6 -benzylguanine in 0.9% saline at zero time followed by a second injection one hour later. In a separate experiment, mice were treated at a dose of 126 mg/kg by injection of about 1 ml of a solution of 5 mg O.sup.6 -benzylguanine in 1 ml of 10% cremophor EL (Sigma) in 0.9% saline, pH 7.0. Control animals received the vehicle alone. Animals were sacrificed by cervical dislocation at various time periods, tissues were removed and frozen at -80.degree. C. until analysis. Crude extracts from tissues for alkyltransferase assay were conventionally prepared as described previously in Pegg et al, J. Biol. Chem., 258, pp. 2327-2333 (1983). Further treatment conditions are described below in Example 5.

The depletion of alkyltransferase in mouse tissues by administration of O.sup.6 -benzylguanine was measured and the results are shown in FIG. 5. CD-1 mice (body wt. about 37 g) were given an intraperitoneal injection of 1 ml of saline containing 0.2 mg of O.sup.6 -benzylguanine (, .largecircle.) or 1 ml of saline alone (, .quadrature.) at zero time and at one h. The mice were killed at the time shown and the alkyltransferase activity was determined in liver (, ) or kidney (.largecircle., .quadrature.) extracts. The means .+-.S.D. for estimations on three mice which were each assayed in duplicate are shown.

Alkyltransferase assay and inactivation. Extracts containing alkyltransferase were incubated with various concentrations of O.sup.6 -benzylguanine or O.sup.6 -methylguanine as indicated. The possible reversal of inactivation was investigated by dialysis against 50 mM Tris, 0.1 mM EDTA and 5 mM dithiothreitol for 16 h following incubation of 25 .mu.M O.sup.6 -benzylguanine in HT29 extract for 2 h at 37.degree. C. Alkyltransferase activity was measured as removal of O.sup.6 -[.sup.3 H]methylguanine from a [.sup.3 H]methylated DNA substrate which was prepared by reacting [.sup.3 H]methylnitrosourea with DNA at 37.degree. C. for 30 min. The DNA was precipitated by adding icecold perchloric acid at a final concentration of 0.25N and hydrolyzed in 0.1N HCl at 70.degree. C. for 30 min as described in Domoradski (1984), supra. The modified bases were separated by conventional reverse-phase HPLC as described in Dolan et al (1986), supra. Protein was determined by the conventional method of Bradford (Anal. Biochem., 72, pp. 248-254 (1976)) and the results expressed as fmol of O.sup.6 -methylguanine released from the DNA per mg of protein.

As shown in Table 1, in vitro exposure of the human O.sup.6 -alkylguanine-DNA alkyltransferase to 0.25 .mu.M O.sup.6 -benzylguanine for 30 min led to a loss of more than 50% of the alkyltransferase activity and exposure to 2.5 .mu.M or higher concentrations completely inactivated the protein.

The loss of alkyltransferase activity was irreversible. Dialysis for 16 h at 4.degree. C. of extracts inactivated by reaction with 25 .mu.M O.sup.6 -benzylguanine led to no restoration of activity. The related compounds, O.sup.6 -(p-chlorobenyl)quanine or O.sup.6 -(p-methylbenyl)quanine, were as active in reducing alkyltransferase as O.sup.6 -benzylguanine itself but 7-benzylguanine had no inhibitory effect and, in agreement with previous reports (Dolan et al, Cancer Res., 45, pp. 6413-6417 (1985); Yarosh et al, Cancer Res., 46, pp. 1663-1668 (1986), O.sup.6 -methylquanine was only weakly active requiring 200 .mu.M concentrations for 60 min to reduce activity by about 50% as shown in Table 1. Inactivation of the alkyltransferase by O.sup.6 -benzylguanine and the parasubstituted analogues was rapid with 50% of the activity lost within 10 min of exposure to 0.5 .mu.M and 90% of the activity lost within 10 min of incubation with 2.5 .mu.M solutions (FIG. 1). It can also be seen in FIG. 1 that the presence of 2.5 .mu.M O.sup.6 -benzylguanine in the assay medium decreased the measured alkyltransferase activity by 57%. This inhibition which occurred in the absence of any preincubation of the alkyltransferase with the drug indicates that O.sup.6 -benzylguanine is an effective competitive inhibitor with respect to the methylated DNA substrate.

Exposure of HT29 cells to O.sup.6 -benzylguanine led to the efficient depletion of alkyltransferase activity (FIGS. 2(A) and 2(B)). Complete loss of activity was seen in response to addition of 5 .mu.M or higher concentrations of this compound to the culture medium for 4 h while 100 .mu.M concentrations of O.sup.6 -methylguanine were needed to achieve a maximal response which was only an 80% reduction in activity (FIG. 2(A)). As illustrated in FIG. 2(B), the depletion was dose dependent in the range of 0 to 2.5 .mu.M for the three O.sup.6 -benzylguanine derivatives while 7-benzylguanine exhibited no effect. This latter observation together with the data in Table 1 establishes that the inhibitory effect of the O.sup.6 -benzylguanines is the result of the attachment of the respective benzyl group to the O.sup.6 -position of the purine and is not a general result of exposure to any benzylated guanine. The data of FIG. 2(B) also indicates that at very low concentrations (i.e. 10 to 100 nM) the p-chlorobenzyl and p-methylbenzyl- derivatives were somewhat more potent than O.sup.6 -benzylguanine but at doses of 0.5 .mu.M or higher all three compounds were equally active.

The time course for alkyltransferase depletion in HT29 cells by both O.sup.6 -benzylguanine and O.sup.6 -methylguanine is illustrated in FIG. 3. The presence of O.sup.6 -benzylguanine at 100 .mu.M concentration in the culture medium caused a complete loss of alkyltransferase activity within 15 min and this was maintained over the entire 8 h period studied. The identical exposure to O.sup.6 -methylguanine produced only an 80% reduction and the maximal effect required 4 h of exposure (FIG. 3). The rate of recovery of alkyltransferase activity 5 was analyzed in HT29 cells by changing the medium to remove the base after exposure to 10.mu.M O.sup.6 -benzylguanine or 400 .mu.M O.sup.6 -methylguanine for 2 h. The alkyltransferase activity remained below 4% of control values for up to 8 h after the period of exposure to O.sup.6 -benzylguanine. In contrast, as previously reported (Dolan et al (1985), supra; Dolan et al, Cancer Rest, 48, pp. 3603-3606 (1988)), after the same time of exposure to O.sup.6 -methylguanine there was a steady rise in alkyltransferase activity which reached 34% at 8h.

The reduction of alkyltransferase in HT29 cells by exposure to 10 .mu.M O.sup.6 -benzylguanine for 2 h led to a very marked increase in the cytotoxicity of either CCNU (FIG. 4(A)) or Clomesone (FIG. 4(B)). Exposure to O.sup.6 -benzylguanine alone showed no toxic effects at doses below 100 .mu.M for 24 h.

In order to be a clinically or experimentally useful tool for manipulating alkyltransferase levels, it is necessary that O.sup.6 -benzylguanine be active in vivo. This was tested by administering the drug to CD-1 mice. Treatment by intraperitoneal injection at a dose of 10.2 mg/kg led to a more than 90% reduction in the alkyltransferase activity in the liver and kidney within 2 h after injection (FIG. 5). The effect was then slowly reversed over the next 15 h. However, when O.sup.6 -benzylguanine was given at a dose of 126 mg/kg (this required that the drug be dissolved in Cremophor EL, Sigma Chemical Co., St. Louis, Mo.) alkyltransferase activity was completely abolished for at least 16 h.

The above Examples demonstrate that O.sup.6 -benzylguanines might be more effective at depleting alkyltransferase levels than any agent studied so far. All of O.sup.6 -benzylguanine derivatives in the Examples were much more active than O.sup.6 -methylguanine or other derivatives such as O.sup.6 -n-butylguanine in terms of concentration needed to inactivate as well as the rate and completeness of alkyltransferase inactivation. Even at low doses of the order of 10 mg/kg, O.sup.6 -benzylguanine completely depleted the alkyltransferase activity in mouse liver and kidneys. Although larger doses were required to achieve longer lasting reductions, it should be noted that a transient decrease is all that is needed to enhance the effectiveness of chloroethylating agents. Furthermore, there were no obvious toxic effects of much larger doses which led to a long lasting reduction in alkyltransferase levels. Treatment with O.sup.6 -benzylguanine may also be a useful way in which to evaluate the role of the alkyltransferase activity in protection against the mutagenic and carcinogenic actions of alkylating agents.

The O.sup.6 -benzylguanine derivatives employed in the present invention may be made into pharmaceutical compositions by accommodation with appropriate pharmaceutically acceptable excipients or carriers or diluents, and may be formulated into preparations in solid, semi-solid, liquid or gaseous forms such as tablets, capsules, powders, granules, ointments, solutions, suppositories, injections, inhalants, and aerosols in the usual ways for their respective route of administration. The following methods and excipients are merely exemplary and are in no way limiting.

In pharmaceutical dosage forms, the O.sup.6 -benzylguanine derivatives employed in the present invention may be used in the form of their pharmaceutically acceptable salts, and also may be used alone or in appropriate association, as well as in combination with other pharmaceutically active compounds.

In the case of oral preparations, the O.sup.6 -benzylguanine derivatives of the present invention may be used alone or in combination with appropriate additives to make tablets, powders, granules or capsules, e.g. with conventional additives such as lactose, mannitol, corn starch or potato starch; with binders such as crystalline cellulose, cellulose derivatives, acacia, corn starch or gelatins; with disintegrators such as corn starch, potato starch or sodium carboxymethylcellulose; with lubricants such as talc or magnesium stearate; and, if desired, with diluents, buffering agents, moistening agents, preservatives and flavoring agents.

Furthermore, the O.sup.6 -benzylguanine derivatives employed in the present invention may be made into suppositories by mixing with a variety of bases such as emulsifying bases or water-soluble bases.

The O.sup.6 -benzylguanine derivatives employed in the present invention may be formulated into preparations for injections by dissolving, suspending or emulsifying them in an aqueous or non-aqueous solvent, such as vegetable oil, synthetic aliphatic acid glycerides, esters of higher aliphatic acids or propylene glycol; and if desired, with conventional additives such as solubilizers, isotonic agents, suspending agents, emulsifying agents, stabilizers and preservatives.

In the cases of inhalations or aerosol preparations, the O.sup.6 -benzylguanine derivatives employed in the present invention in the form of a liquid or minute powder may be filled up in an aerosol container with gas or liquid spraying agents, and if desired, together with conventional adjuvants such as humidifying agents. They may also be formulated as pharmaceuticals for non-pressured preparations such as a nebulizer or an atomizer.

The amount of O.sup.6 -benzylguanine derivatives employed in the present invention to be used varies according to the degree of the effective amount required for treating tumor cells. A suitable dosage is that which will result in a concentration of the O.sup.6 -benzylguanine derivatives in the tumor cells to be treated which results in the depletion of AGT activity, e.g. about 1-2000 mg/kg prior to chemotherapy and preferably 10-800 mg/kg prior to chemotherapy.

Unit dosage forms for oral administration such as syrups, elixirs, and suspensions wherein each dosage unit (e.g., teaspoonful or tablespoonful) contains a predetermined amount of the O.sup.6 -benzylguanine derivative employed in the present invention can be combined with a pharmaceutically acceptable carrier, such as Sterile Water for Injection, USP, or by normal saline.

The O.sup.6 -benzylguanine derivatives employed in the present invention can be administered rectally via a suppository. The suppository can include vehicles such as cocoa butter, carbowaxes and polyethylene glycols, which melt at body temperature, yet are solidified at room temperature.

The O.sup.6 -benzylguanine derivatives employed in the present invention can be administered transdermally in an appropriate vehicle or salt or converted to a salt. Adsorption may be aided by the use of an electric current or field.

The O.sup.6 -benzylguanine derivatives employed in the present invention may be administered with an appropriate vehicle for buccal or sublingual administration.

The O.sup.6 -benzylguanine derivatives employed in the present invention can be utilized in aerosol formulations to be administered via inhalation. The O.sup.6 -benzylguanine derivatives can be formulated into pressurized acceptable propellants such as dichlorodifluoromethane, propane, nitrogen and the like.

The term "unit dosage form" as used herein generally refers to physically discrete units suitable as unitary dosages for human and animal subjects, each unit containing a predetermined quantity of the O.sup.6 -benzylguanine derivatives calculated in an amount sufficient to produce the desired effect in association with a pharmaceutically acceptable diluent, carrier, excipient or vehicle. The specifications for the novel unit dosage forms of the present invention depend on the particular compound employed and the effect to be achieved, and the pharmacodynamics associated with each compound in the host.

The pharmaceutically acceptable excipients, for example, vehicles, adjuvants, carriers or diluents are readily available to the public.

Any necessary adjustments in dose can be readily made to meet the chemotherapeutic treatment requirements and adjusted accordingly by the skilled practitioner.

Alkylating agents, such as chloroethylating agents, may be administered using conventional techniques such as those described in Wasserman et al, Cancer, 36: pp. 1258-1268 (1975); and Physicians' Desk Reference, 44th ed., Edward R. Barnhart publisher (1990). For example, 1,3-bis(2-chloroethyl)-1-nitrosourea (carmustine or BCNU, Bristol-Myers, Evansville, Ind.) may be administered intravenously at a dosage of from about 150 to 200 mg/m.sup.2 every six weeks. Another alkylating agent, 1-(2-chloroethyl)-3-cyclohexyl-l-nitrosourea (lomustine or CCNU, Bristol-Myers), may be administered orally at a dosage of about 130 mg/m.sup.2 every six weeks. Other alkylating agents may be administered in appropriate dosages via appropriate routes of administration known to skilled medical practioners.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.