Aromatic hydroxylation of ellipticine in rats: lack of an NIH shift

CYP2D6-mediated catalysis of tamoxifen aromatic hydroxylation with an NIH shift: similar hydroxylation mechanism in chicken, rat and human liver microsomes

1. 4-Tritiated-tamoxifen (4-[(3)H]-tamoxifen) and 4-deuterated-tamoxifen (4-[(2)H]-tamoxifen) were synthesized to examine tamoxifen metabolism by human P450 (CYP) forms and also for the possibility of determining tamoxifen-4-hydroxylation in humans in vivo. 2. Liver microsomes from several species and cDNA-expressed human P450s were incubated with 4-[(3)H]-tamoxifen and the reaction monitored by assaying 4-hydroxytamoxifen (4-OH-tam) and (3)H(2)O formed. However, tamoxifen-4-hydroxylation did not generate stoichiometric amounts of (3)H(2)O and the expected unlabelled 4-OH-tam but instead yielded radiolabelled 4-OH-tam, apparently from [(3)H]-migration to the ortho-position, referred to as the NIH shift. 3. CYP2D6 was the prime catalyst of tam-4-hydroxylation, whereas CYP2B6, 2C9 and 2C19 yielded only low levels of 4-OH-tam; nevertheless, in all cases the 4-OH-tam was radioactive, apparently resulting from reactions involving an NIH shift. 4. Chicken liver microsomal preparation, being catalytically the most active in tamoxifen-4-hydroxylation, was incubated with deuterated tamoxifen (4-[(2)H]-tamoxifen) in order to determine whether an NIH shift occurs. Ion-trap mass-spectrometry of the HPLC-purified 4-OH-tam, from that incubation, indicated about 60% of [(2)H]-retention in 4-OH-tam, signifying an NIH shift. These findings indicate that the aromatic hydroxylation of tamoxifen does not entail hydroxyl insertion with an Sn2-displacement of hydrogen or a hydrogen isotope ((2)H or (3)H), but apparently involves epoxidation followed by migration of the (3)H, (2)H or (1)H to the ortho-position, and dissociation of the (1)H in preference to (3)H or (2)H, i.e. retention of the hydrogen isotope appears to be related to the bond strengths: C-(3)H>C-(2)H>C-(1)H.

Chemiexcitation in the peroxidative metabolism of N-methylcarbazole: mechanistic implications

The peroxidative metabolism of N-methylcarbazole emits light independently of the presence of oxygen. It is likely that two chemiexcited transients are formed by electron transfer to the activated peroxidase, the cation radical by one electron transfer and a cation biradical by two electron transfer consistent with the failure to observe horseradish peroxidase-II in the steady state of the reaction. In the spectral range investigated (390-700 nm) the observed emission (570-700 nm) is ascribed to the biradical, as the latter is equivalent to an excited state of the postulated iminium cation. While lipoxygenase has no effect upon N-methylcarbazole, it markedly enhances the emission if peroxidase is present. This effect requires oxygen and is ascribed to an excited product formed by lipoxygenase acting upon an intermediate hydroperoxide of the aerobic process promoted by peroxidase. Our results are of importance on two counts. First they extend to N-methylcarbazole the formation of excited species in the peroxidative metabolism of important xenobiotics. Second, the mechanistic information they provide supports the scheme of metabolism postulated by Kedderis et al. (1986, J. Biol. Chem. 261, 15910-15914).

Antitumor activity, pharmacology, and toxicity of ellipticines, ellipticinium, and 9-hydroxy derivatives: preliminary clinical trials of 2-methyl-9-hydroxy ellipticinium (NSC 264-137)

Ellipticine and some derivatives are highly cytotoxic substances which kill L1210 cells at concentrations ranging form 10(-8) to 10(-6)M. Some compounds in this series bind with high affinity to DNA (affinity constant between 10(7) M-1 and 10(5) M-1) by intercalation between base pairs. The antitumoral properties of these derivatives are thought to be related to their DNA-binding ability. Both 9-hydroxylation of ellipticine and quaternarization of 2-pyridinic nitrogen tend to increase DNA binding and antitumor activity. 2-Methyl-9-hydroxyellipticine (NSC 264-137) was selected for a phase I and later for a phase II trial in human cancer. This drug does not affect blood cell counts in animals or in man. It is not mutagenic in the Ames' test nor teratogenic in mice, but is endowed with anti-inflammatory properties and induces a marked decrease of motoricity in mice. Transient bradycardia and decrease of blood pressure are the most noticeable cardiovascular effects in dogs. This compound administered at 80-100 mg/m2/week in 1-h intravenous (IV) infusion induces objective remissions in about 25% of patients suffering from advanced breast cancer refractory to all other treatment. These remissions, which occurred after 3-4 weeks, lasted for 1-18 months. This drug seems particularly to improve the condition of patients suffering from oesteolytic breast cancer metastasis. Activity against anaplastic thyroid carcinoma and ovarian carcinoma has also been observed in some cases. Toxic side effects are nausea and vomiting (one-third of the patients), hypertension (less than 10% of the patients), muscular cramp (one-third of the patients), fatigue which can be very pronounced (in most patients after 3 months of treatment), mouth dryness, and mycosis of the tongue and esophagus (less than 20% of the patients).

Comparative cytotoxic and antitumoral effects of ellipticine derivatives on mouse L 1210 leukemia

Twelve derivatives of the antitumoral alkaloid ellipticine (E) and ellipticinium were assayed in vitro on cultured L 1210 cells. These drugs possess varying abilities to decrease the cell growth rate in a 1--1000-fold range. Some of them have a highly cytotoxic effect in the 10(-8)--10(-6) M range. Non-specific intracellular damages are produced: multilobation of nuclei, occurrence of numerous lipid granules, diminution of the size and increase in the number of mitochondrial profiles and several modifications of the internal architecture of mitochondria. 2-Methyl-9-hydroxyellipticinium (2-CH3-9-OHE) was submitted to a bioassay; it inactivates the tumorigenic potency of the cells exposed to it, when they are grafted back into mice in the same dose range which reduces in vitro the growth rate of the cells. A fairly good correlation holds between the in vitro and in vivo (antitumor effect) assays, offering a possible prescreening test for a cheaper and rapid evaluation of chemotherapeutic activity of these compounds. The results stress again the importance of the 9-hydroxy substitution in these series for improving the anticancer efficiency. The nature of the biochemical target of E and derivatives is discussed according to our data.