When the Dose Is Not "Holding" the Patient
A variety of complaints may introduce the case for more methadone, for example, "I wake up sick; I have a strong urge to fix; I am fixing." There are a number of reasons why the patient who was stable may be having problems in relation to dose.
Perhaps the most frequent cause is the ingestion of other substances, especially alcohol. Any drug that stimulates the liver's microsomal enzyme-oxidizing system may accelerate the metabolism of methadone. Barbiturates and other sedative-hypnotics may also produce this effect.
Specific drugs known to accelerate methadone metabolism, and at times, to precipitate AS, include rifampin (Tong et al. 1981), phenytoin (Dilantin) (Kreek 1978), and carbamazepine (Tegretol) (see table 4). A.J. Saxon (1989) suggested that valproic acid, unlike other anticonvulsants, has no effect on methadone metabolism. Although this opinion was based on only two cases, consideration of valproic acid would be justified when the clinician faces a choice between seizures or abstinence.
Inadvertent administration of opioid agonist/antagonist drugs can also precipitate AS by an entirely different mechanism (see table 4).
Environmental changes and other stresses can cause the patient to perceive that the dose is not adequate and to experience increased drug craving. Events that increase the availability of drugs, such as another addict moving in at home or a "connection" opening nearby, can intensify craving. Dose increases may be quite appropriate in such cases, although efforts should focus on resolving the offending situation rather than relying on more methadone. Conversely, diminished availability of drugs, as may occur in prison or jail, may diminish drug craving.
In the absence of medication or environmental contributions and polysubstance abuse in an apparently destabilized methadone maintenance patient, plasma level determinations should be considered. Figure 3 is an approximation of a typical 24-hour blood plasma curve based on established steady-state maintenance, with the zero-hour dose approximately 24 hours after the previous dose. The data for the figure are derived by averaging a series by Inturrisi and Verebey (1972) and one by Kreek (1973). Both series clearly demonstrated that the peak level is less than twice the trough level. This ratio is important for the clinician who is interpreting methadone blood plasma levels.
At present, 150 ng/ml is generally accepted as the lowest level that will maintain steady-state effect (Dole 1988). The optimum 24-hour mean plasma level may be more in the 400-ng/ml range (Goldstein pers. com. 1991; Kreek 1973; Tennant 1987; Wolff et al. 1991). Loimer and colleagues (1991) suggest that "methadone plasma concentrations of 400 ng/ml necessary to suppress any further opiate action and to provide stabilized maintenance." The optimum dose is the level at which there is adequate methadone to provide constant availability to the opiate receptors. The data for the lower curve in figure 3 are based on experience with an actual patient on 80 mg of methadone daily who persistently complained of waking up sick and having drug hunger. This patient responded to an increase in dose.
Figure 4 illustrates experience with a patient with low zero- and 24-hour methadone plasma levels with a peak that was within normal values but was high in relation to the very low trough levels. In the figure, the peak is more than three times the nadir and the absorption and elimination portions of the curve are much steeper, indicating a rapid change in state. In such a clinical situation, it is likely that the rate of change is as important as, or more important than, the numeric values themselves (see fig. 4).
If the dose is increased in an effort to bring up the 0-24-hour level, the peak level may be excessive, thus exaggerating the abnormal curve. Assuming that the cause for the rapid elimination is not apparent (drugs, urinary pH, etc.), a "split dose" may be indicated to avoid having the patient somewhat overmedicated for a few hours, feeling normal for a while, and still feeling bad or waking up sick later. Figure 4 shows the split-dose desired responses in two so-called "fast Maintenance metabolizers." In both cases, the total dose is the same as for a 24-hour period and the area under the curve is essentially unchanged. What changes is that both the low nadir and the high peak are eliminated, resulting in a smoother clinical response associated with from the flattening of the curve.
Excretion of methadone via the kidneys is pH dependent. Studies have shown that by altering the pH from very acid to very alkaline, half-life of methadone may vary from less than 18 hours than 40 hours (Nilsson et al. 1982). The clinical significance of more modest variation in urinary pH has need anything else for pain. This is not been demonstrated but probably deserves attention in evaluating the patient who is not getting a 24-hour effect from the methadone.