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An even more focused direction of this work is to explore the pathway resulting from oxidative stress. Exposure of cells to toxic chemicals can result in reduced glu-
8.13 Enzyme Induction (CYP3A4) and Drug Design 1117
tathione (GSH) depletion, generation of free radicals, and binding to critical cell constituents. This binding and resultant chemical stress is usually followed by a concerted cellular response aimed at restoring homeostasis, although the precise initial stimulus for the response is unclear. One component of this stress response is the upregulation of y-glutamylcysteine synthetase (y-GCS) and the preceding molecular events involved in its regulation. C-jun and c-fos mRNA (mRNA) levels and activator protein 1 [AP-1] have been found to be sensitive markers for a number of toxicants
8.13 Enzyme Induction (CYP3A4) and Drug Design
Although largely an adaptive response and not a toxicity enzyme induction, the interaction of cytochrome P450 in particular, with a drug is undesirable, as it may affect the efficacy of the drug or co-administered drugs. The number of clinically used drugs which induce P450 enzymes is, in fact, quite limited. However, in certain disease areas (AIDS, epilepsy) many of the drugs used, whether for primary or secondary indications, have the potential for enzyme induction. Induction is often seen pre-clinically, due to the elevated dose levels used, but this potential rarely transfers to the clinical situation .
No clear SAR emerges for induction, nor are any particular groups or functions implicated as shown by the diverse structures of the known CYP3A4 inducers (Figure 8.29). Structures are diverse but most are lipophilic as defined by a positive calculated log P value.
A critical factor in P450 induction in the clinic, based on drugs known to induce P450, is the question of dose size. The major inducible form of P450 in man is CYP3A4. The drugs that induce CYP3A4 are given in high doses, often around 500-1000 mg day-1 (Table 8.3). These result in total drug concentrations in the
Tab. 8.3 Dose, total and free plasma concentrations for clinical CYP3A4 inducers.
Dose (mg day-1) Cp ДО) Cp free (ЦМ)
Carbamazepine 400-1200 12 3.6
Phenytoin 350-1000 54 5
Rifampicin 450-600 12 4
Phenobarbitone 70-400 64 32
Troglitazone 200-600 7 0.01
Efavirenz 600 29 0.3
Nevirapine 400 31 12
Moricizine 100-400 3 0.5
Probenicid 1000-2000 350 35
Felbamate 1200-3600 125 95
118 8 Toxicity
Fig. 8.29 Structures of known clinical CYP3A4 inducers: nevirapine (A), troglitazone (B), phenobarbitone (C), efavirenz (D), probenicid (E), phenytoin (F), moricizine (G), felbamate (H), rifampicin (I) and carba-mazepine (J).
10-100 juM range or approximately an order of magnitude lower than that expressed as a free drug concentration (Table 8.3). The concentrations equate closely to the therapeutic plasma concentrations presented in Table 8.3. These data both reflect the relatively weak affinity of the inducing agents and the need for high concentrations or doses. The high clinical concentrations reflect the weak potency of the drugs. For instance the Na+ channel blockers have affinities of 3, 9 and 25 |uM (moricizine, phenytoin and carbemazepine, respectively). With the anti-infectives there is the need to dose to the IC95 level or greater. Thus, although efavirenz is a potent inhibitor of wild-type RT HIV (Ki = 3 nM), there is a need to go to higher concentrations to reach the IC95 for the virus and also to treat for possible mutants.
In contrast to these concentrations many clinically-used drugs, which are non-inducers are effective at doses up to two orders of magnitude lower. The need for high doses has other undesirable complications. As outlined above dose size is important in toxicity and enzyme inducers show a high level of adverse drug reactions affecting such organs and tissues as the liver, blood and skin (Table 8.4).
This statement is somewhat at odds with the conventional view that idiosyncratic toxicology is dose-size independent. Idiosyncratic reactions are thought to result from an immune-mediated cell injury triggered by previous contact with the drug. The toxicity may appear after several asymptomatic administrations of the com-
8.13 Enzyme Induction (CYP3A4) and Drug Design 1119 Tab. 8.4 Clinical toxicities and side-effects of P4503A4 inducers.
Carbamazepine Aplastic anaemia, agranulocytosis,
skin rash, hepatitis
Phenytoin Agranulocytosis, skin rash, hepatitis
Rifampicin Shock, haemolytic anaemia, renal failure
Phenobarbitone Aplastic anaemia, agranulocytosis,
Troglitazone Hepatic toxicity
Efavirenz Hepatitis, skin rash
Nevirapine Hepatitis, skin rash
Probenicid Aplastic anaemia, hepatic necrosis
Felbamate Aplastic anaemia
pound (sensitization period) and is not perceived as dose dependent. For instance when the relationship between the occurrence of adverse side-effects and the use of anti-epileptic drugs was examined, there was no definite dose- or serum concentration-dependent increase in the incidence of side-effects. In fact on closer examination idiosyncratic toxicology and dose size seem firmly linked. Not in the terms of a single drug used over its clinical dose range as above, but that adverse reactions occur more often with high dose drugs. Aside from the examples above an excellent example is clozapine and its close structural analogue olanzapine (Figure 8.30). Clozapine is used clinically over the dose range of 150-450 mg and its use is associated with agranulocytosis. Olanzipine is used clinically at 5-10 mg and is associated with a negligible risk of agranulocytosis. As outlined in Section 8.4 both compounds could potentially be activated to form reactive intermediates such as nitrenium ions.