N-methyl antitumour agents. A distinct class of anticancer drugs?

The abundant DNA adduct N 7-methyl deoxyguanosine contributes to miscoding during replication by human DNA polymerase η

Aside from abasic sites and ribonucleotides, the DNA adduct N ⁷-methyl deoxyguanosine (N⁷ -CH₃ dG) is one of the most abundant lesions in mammalian DNA. Because N⁷ -CH₃ dG is unstable, leading to deglycosylation and ring-opening, its miscoding potential is not well-understood. Here, we employed a 2'-fluoro isostere approach to synthesize an oligonucleotide containing an analog of this lesion (N⁷ -CH₃ 2'-F dG) and examined its miscoding potential with four Y-family translesion synthesis DNA polymerases (pols): human pol (hpol) η, hpol κ, and hpol ι and Dpo4 from the archaeal thermophile Sulfolobus solfataricus We found that hpol η and Dpo4 can bypass the N⁷ -CH₃ 2'-F dG adduct, albeit with some stalling, but hpol κ is strongly blocked at this lesion site, whereas hpol ι showed no distinction with the lesion and the control templates. hpol η yielded the highest level of misincorporation opposite the adduct by inserting dATP or dTTP. Moreover, hpol η did not extend well past an N ⁷-CH₃ 2'-F dG:dT mispair. MS-based sequence analysis confirmed that hpol η catalyzes mainly error-free incorporation of dC, with misincorporation of dA and dG in 5-10% of products. We conclude that N ⁷-CH₃ 2'-F dG and, by inference, N ⁷-CH₃ dG have miscoding and mutagenic potential. The level of misincorporation arising from this abundant adduct can be considered as potentially mutagenic as a highly miscoding but rare lesion.

Replication of the 2,6-diamino-4-hydroxy-N(5)-(methyl)-formamidopyrimidine (MeFapy-dGuo) adduct by eukaryotic DNA polymerases

N(6)-(2-Deoxy-d-erythro-pentofuranosyl)-2,6-diamino-3,4-dihydro-4-oxo-5-N-methylformamidopyrimidine (MeFapy-dGuo) has been identified as a stable DNA adduct that arises from the reaction of DNA with a variety of methylating agents. Since this lesion persists in DNA and may contribute to the overall mutagenesis from electrophilic methylating agents, the MeFapy-dGuo lesion was incorporated into oligonucleotides, and its replication bypass was examined in vitro with a panel of eukaryotic high fidelity (hPols α, β, and δ/PCNA) and translesion (hPols η, κ, ι, Rev1, ν, and yPol ζ) polymerases to address its miscoding potential. The MeFapy-dGuo was found to be a strong block to the high fidelity polymerases at either the insertion or the extension step. Efficient translesion synthesis was observed for hPols η and κ, and the combined activities of hRev1 and yPol ζ. The nucleotide sequences of the extension products were determined by mass spectrometry. The error-free extension product was the most abundant product observed for each polymerase. Misreplication products, which included misinsertion of Thy, Gua, and Ade opposite the MeFapy-dGuo lesion, as well as an interesting one-nucleotide deletion product, were observed when hPols η and κ were employed; these events accounted for 8-29% of the total extension products observed. The distribution and abundance of the misreplication products were dependent on the polymerases and local sequence context of the lesion. Collectively, these data suggest that although MeFapy-dGuo adducts represent a relatively minor proportion of the total alkylated lesions, their miscoding potentials could significantly contribute to genomic instability.

Reappraisal of the use of procarbazine in the treatment of lymphomas and brain tumors

Procarbazine HCl is a 'nonclassical' oral alkylating anticancer agent that was first synthesized in the late 1950s. It has been used in the treatment of many cancers, but its main use is in the treatment of Hodgkin's lymphoma and brain tumors and, to a lesser extent, Non-Hodgkin's lymphoma and primary central nervous system lymphoma. Procarbazine is a prodrug that undergoes metabolic transformation into active intermediates that are thought to inhibit DNA, RNA, and protein synthesis. Early use of procarbazine in combination with mechlorethamine, vincristine, and prednisone (MOPP) was effective in the treatment of advanced Hodgkin's lymphoma, but late toxic effects such as secondary cancer and infertility led to its replacement by other regimens. However, its recent reintroduction in the dose-intensified BEACOPP (bleomycin, etoposide, doxorubicin, cyclophosphamide, vincristine, procarbazine, and prednisone) regimen has yielded very promising findings. Procarbazine alone, or more commonly combined in the PCV (procarbazine, lomustine [CCNU], and vincristine) regimen, is also effective in treating gliomas comprising astrocytomas, glioblastomas, and oligodendrogliomas. The most common side effects of procarbazine are gastrointestinal disturbances, myelosuppression, and central nervous system effects. In conclusion, the use of procarbazine in combination with other drugs means that it remains a major anticancer drug in the management of Hodgkin's lymphoma and gliomas.

O6-methylguanine DNA methyltransferase deficiency and response to temozolomide-based therapy in patients with neuroendocrine tumors

PURPOSE

Recent studies suggest that temozolomide has activity in neuroendocrine tumors. Low levels of the DNA repair enzyme, O(6)-methylguanine DNA methyltransferase (MGMT), are associated with sensitivity to temozolomide in other tumor types. We evaluated the prevalence of MGMT deficiency in neuroendocrine tumors and correlated MGMT deficiency with treatment response to temozolomide-based regimens.

EXPERIMENTAL DESIGN

The prevalence of MGMT deficiency, measured by immunohistochemistry, was assessed in 97 archival neuroendocrine tumor specimens. Rates of treatment response and survival were next evaluated in a cohort of 101 consecutive neuroendocrine tumor patients who had received treatment with a temozolomide-based regimen at one of three institutions. MGMT expression was directly correlated with treatment response in 21 patients who had available tumor tissue and response data.

RESULTS

In archival specimens, MGMT deficiency was observed in 19 of 37 (51%) pancreatic neuroendocrine tumors and 0 of 60 (0%) carcinoid tumors (P < 0.0001). In the clinical cohort, 18 of 53 (34%) patients with pancreatic neuroendocrine tumors but only 1 of 44 (2%) patients with carcinoid tumors (P < 0.001) experienced a partial or complete response to temozolomide-based therapy. Among 21 patients with evaluable tumor tissue who had also received treatment with temozolomide, 4 of 5 patients with MGMT-deficient tumors (all pancreatic neuroendocrine tumors) and 0 of 16 patients with tumors showing intact MGMT expression responded to treatment (P = 0.001).

CONCLUSIONS

MGMT deficiency, measured by immunohistochemistry, is more common in pancreatic neuroendocrine tumors than in carcinoid tumors as is treatment response to temozolomide-based therapy. Absence of MGMT may explain the sensitivity of some pancreatic neuroendocrine tumors to treatment.

Determination of terephthalic acid isopropylamide in urine with a liquid chromatography/mass spectrometry (LC/MS) method

A sensitive and simple liquid chromatography/mass spectrometry (LC/MS) method was developed for the determination of terephthalic acid isopropylamide, the final metabolite of procarbazine in human urine. A solid-phase extraction with C(18) cartridges was used followed by LC/MS with a single mass spectrometer (SSQ 7000 from Finnigan). Terephthalic acid isobutylamide was the internal standard. The quantification limit was 30 ng/mL in urine (6 x noise). This assay was applied for drug monitoring of terephthalic acid isopropylamide in urine after oral administration of procarbazine in children and adolescents with Hodgkin lymphomas.

Procarbazine for non-Hodgkin's lymphoma

Procarbazine hydrochloride is an oral alkylating agent with activity against lymphoma. It is most commonly used in the treatment of Hodgkin's disease. The use of procarbazine-containing chemotherapeutic regimens in non-Hodgkin's lymphoma fell out of favor with the advent of CHOP. We report two patients with relapsed and/or refractory follicular lymphoma that achieved a complete and durable remission with a prolonged course of daily procarbazine.

Plasma kinetics of procarbazine and azo-procarbazine in humans

The plasma kinetics of procarbazine (PCB) and its major metabolite azo-procarbazine (azo-PCB) were systematically investigated in humans for the first time. Eight therapy-refractory tumor patients with normal liver and renal function were given a single oral dose of 300 mg PCB hydrochloride as a drinking solution under fasting conditions. With the exception of the single i.v. administration of 10 mg ondansetron hydrochloride immediately before the administration of PCB, the patients were free of any co-medication 4 weeks before and during the study. PCB and azo-PCB were determined by a specially developed HPLC-UV method. PCB was absorbed very rapidly. Mean maximum plasma concentration was 12.5 min. A high elimination rate of PCB from plasma was found. The mean apparent oral systemic clearance and the plasma elimination half-life were estimated at 35.8 l/min and 9.2 min, respectively. Considerable amounts of azo-PCB are found in the plasma of the eight tumor patients. The mean Cmax and AUC ratios of azo-PCB/PCB were estimated at 5.5 and 45.2. Azo-PCB is formed very rapidly from PCB, but eliminated much more slowly from plasma than PCB. Considerable interindividual differences in the conversion rate of azo-PCB to its further metabolites were observed which should have consequences for the individual tumor therapeutic efficiency of PCB. No toxic side-effects or symptoms such as nausea or vomiting were observed during the entire study.

In vivo administration of O(6)-benzylguanine does not influence apoptosis or mutation frequency following DNA damage in the murine intestine, but does inhibit P450-dependent activation of dacarbazine

Clinically relevant cancer chemotherapeutic alkylating agents such as temozolomide and dacarbazine induce apoptosis and are mutagenic via the formation of O(6)-alkylguanine adducts in DNA. The DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) functions by dealkylating such adducts and can thus prevent apoptosis and mutagenesis. In attempts to maximize the clinical effectiveness of these alkylating agents, inhibitors of AGT such as O(6)-benzylguanine (BeG) have been developed. We show here that within murine small intestinal crypt cells, BeG administration does not alter the apoptotic response to the direct-acting methylating agents N-methyl-N-nitrosurea (MNU), temozolomide and N-methyl-N'-nitro-N-nitrosoguanidine. Furthermore, we show that BeG pretreatment fails to elevate the mutation frequency at the murine Dlb-1 locus following exposure to MNU. Consistent with these results, we show that intestinal AGT activity is effectively abolished by administration of 100 mg/kg temozolomide, even in the absence of BeG. In contrast, pretreatment with BeG transiently abolished the apoptotic response to the methylating prodrug dacarbazine. Activation of dacarbazine to its reactive intermediate has previously been shown to be cytochrome P450 dependent and we show here that pretreatment of mice with the cytochrome P450 inhibitor metyrapone also inhibits dacarbazine-induced apoptosis. Thus BeG increases neither the prevalence of apoptosis nor mutation frequency in the murine small intestine, but is capable of inhibiting P450-dependent prodrug activation. The positive implication from this study is that BeG treatment may not exacerbate the toxic and mutagenic effects of methylating agents within normal cells, although it may engender other adverse reactions through the suppression of cytochrome P450-dependent processes.

A Cancer Research Campaign (CRC) phase II trial of CB10-277 given by 24 hour infusion for malignant melanoma

The decarbazine analogue CB10-277 has been investigated for anti-tumour activity in a phase II study on malignant melanoma. Treatment was administered as a slow infusion of 12,000 mg m-2 over 24 h and repeated every 3 weeks. A total of 28 patients were entered into the study, of whom 23 were eligible for review. A total of 64 courses was given. There was one objective partial response in 22 patients assessable for response. The major toxicities were leucopenia and thrombocytopenia. CB10-277 in this schedule therefore does not appear to have major activity in melanoma.

Effects of elevated temperature on misonidazole O-demethylation by mouse liver microsomes: kinetic and stability studies of a model mixed-function oxidase reaction

We investigated the effects of a range of temperatures (33 degrees-44 degrees C) on the stability and kinetics of C3H mouse liver microsomal misonidazole (MISO) O-demethylase in vitro. Microsomal O-demethylase activity was stable for 60 min at 37 degrees C and for 30 min at 41 degrees C but was steadily inactivated with longer incubation times. Inactivation at 44 degrees and 47 degrees C was exponential, with half-lives of 41 and 11 min, respectively. MISO O-demethylation followed Michaelis-Menten kinetics from 33 degrees to 44 degrees C. The apparent Vmax for desmethylmisonidazole (Ro 05-9963) formation was decreased by 32% (from 2.14 to 1.47 nmol min-1 mg-1 protein) with a 4 degrees decrease from 37 degrees to 33 degrees C. An increase of 4 degrees from 37 degrees to 41 degrees C enhanced the Vmax by 47%, but there was only an additional 9% increase for a further 3 degrees rise to 44 degrees C. Apparent Km values were unaltered at about 1.6 mM. These results show that elevated temperatures in the clinically relevant hyperthermia range (41 degrees-44 degrees C) can enhance a model mixed-function oxidase reaction in vitro. Such effects may be important for the metabolism, activity and toxicity of anticancer drugs combined with hyperthermia in vivo.