Biological treatments

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CHAPTER 13 Biological treatments

Drug interactions

Most medications are metabolised, at least to some extent, in the liver by the cytochrome P450 enzyme group (CYP enzymes) and this includes most psychotropic medications. A number of psychotropic medications inhibit or induce isoenzymes of this enzyme group creating potentially significant changes in the blood levels of other medications. Inhibition will promote increased levels and induction will promote decreased levels. These effects need to be considered in the prescribing of medication combinations and must be considered before prescribing. CYP polymorphisms and levels vary from person to person, and also over different racial groups. Genetic testing is now available to determine the profile of these polymorphisms if there are specific clinical indications (see Tables 13.1, 13.2 and 13.3).

Some specific examples are noted below. Pharmacists can be very helpful with advice, computerised prescribing programs include warnings about potential interactions, and the internet site ‘MedWatch’ is very useful.

Cigarette smoking induces CYP1A2 and therefore blood levels of some medications (e.g. clozapine) may be lower in smokers than expected.

ABC membrane transport proteins (ATP-bound cassette membrane proteins) are heavily involved in the regulation of transport of multiple substances across cell membranes, including the gut, blood-brain barrier, placenta, kidneys and other organs. P-glycoprotein (Pgp) is one such membrane transport protein which has a significant role in the regulation of transport of medications and other substances and therefore pharmacokinetics (including drug interactions). Most psychotropic medications cross the blood-brain barrier by passive diffusion through their lipophilic characteristics, but Pgp can modify transfer against concentration gradients. A number of medications inhibit Pgp and therefore have interactive effects on the action of other medications. Examples of such inhibitors include most selective serotonin reuptake inhibitors (SSRIs) and serotonergic and noradrenergic reuptake inhibitors (SNRIs) (with the currently known exception of desmethylvenlafaxine), atypical antipsychotics such as risperidone and olanzapine (less so paliperidone) and a variety of other medications. The tendency of Pgp to have common substrates with the CYP3A group of enzymes can also cause alterations in the bioavailability of several medications. Finally, various neurological diseases alter Pgp activity at the blood-brain barrier.

The combination of medications is a common and often necessary element of clinical practice in medicine. However, it always requires attention to possible interactions, which can be complex and very significant. At the same time, not all combinations are problematic and many interactions are trivial in effect.

The combination of antidepressants in the treatment of depressive disorders is an area of some controversy and any combinations should be approached cautiously and with attention to relative effects on CYP enzymes and Pgp. Most medications are carried in the blood bound to proteins. As many share the same carrier proteins, competition alters the level of ‘free’ drug and therefore effect. Alterations in the levels of warfarin, for example, are commonly important (Box 13.1).

Absorption

Medication absorption and therefore bioavailability is affected by:

Age

The very young and the aged exhibit differences in fat/lean body ratios, medication absorption from the gut, hepatic and renal function, and tolerance of side effects from medications (see Chs 16 and 17).

Specific medications

The categories used to classify medications are chosen by the manufacturer when first applying for registration with regulatory authorities. This means that medications may have multiple applications but be ‘known’ by their initial categorisation. Dose ranges given are daily and are suggested for ‘routine’ applications, but exceptions may arise. Side effects listed are those encountered commonly in clinical practice and those which are of particular seriousness. More detailed and comprehensive lists of side effects can be obtained from the ‘References and further reading’ and various printed and electronic prescribing guides.

Most psychotropic medications reduce the seizure threshold, but some have more potent effects than others and will be highlighted in the text.

Anxiolytics

Antidepressants

General comments

BOX 13.5 Clinical notes on antidepressants

Below are suggestions for the choice of antidepressants in common clinical situations, based upon research and personal experience/preference. The list is not exhaustive and clinical judgment must be used in each case, particularly when there is a past history of antidepressant use. When treating depressive disorders, begin with the lowest manufactured dose and increase after 2 weeks if the response is inadequate. When treating anxiety disorders, begin with half the lowest manufactured dose (where practicable) and combine with an anxiolytic for the first week (except NASAs and TCAs). Anxiety disorders will usually require an increase in dose after 2 weeks to achieve an adequate response. Suggestions are:

Selective serotonin reuptake inhibitors (SSRIs)

These include citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine and sertraline (see Table 13.5).

Serotonergic and noradrenergic reuptake inhibitors (SNRIs)

These include desvenlafaxine, duloxetine and venlafaxine. Details of dose range, half-lives and adverse effects can be found in Table 13.6.

Monoamine oxidase inhibitors (MAOIs)

These include moclobemide, phenelzine and tranylcypromine (see Tables 13.8, 13.9 and 13.10).

Adverse effects

TABLE 13.9 MAOI dietary restrictions

High tyramine content: not permitted Moderate tyramine content: limited amounts are safe Low tyramine content: permissible and safe

TABLE 13.10 MAOI dangerous combinations

Amphetamines
Pethidine
Opioids
Lithium
Serotonergic antidepressants
Hypericum perforatum
Tryptophan

Antipsychotics

Adverse effects

BOX 13.6 Extrapyramidal side effects (EPS)

Acute forms of EPS

EPS may include:

First generation antipsychotics (neuroleptics)

These include chlorpromazine, fluphenazine, flupenthixol, haloperidol, pericyazine, trifluoperazine and zuclopenthixol (see Tables 13.11 and 13.12). Practical notes regarding the use of first generation antipsychotics are summarised in Box 13.7.

BOX 13.9 Clinical notes on antipsychotics

The starting dose will usually be the lowest manufactured dose, with any increase determined by the response over days to weeks.

Second generation antipsychotic medications are currently often used in preference to first generation antipsychotics because of the lower risk of EPS and potential benefits upon negative and cognitive symptoms. Asenapine may have some advantages in the treatment of negative and cognitive symptoms. Weight gain, promotion of diabetes mellitus (type 2), hyperlipidaemias and metabolic syndrome from some of these medications are reasons for caution in practice.

First generation antipsychotics are cheaper than second generation medications and may be preferred for this reason.

Ziprasidone is best absorbed if taken with food.

Antipsychotics may prolong cardiac conduction (QTc interval) and examples include haloperidol, quetiapine, ziprasidone and sertindole. The risk is increased by concurrent use of some antiarrhythmic medications, hypokalaemia, hypomagnesaemia and high dose. If there is any doubt, an electrocardiograph should be performed and serum electrolyte levels checked. If gastrointestinal disorders, including severe food restriction in anorexia nervosa, are present then a serum magnesium should also be obtained. A QTc interval of < 500 ms is considered reasonable, but for safety < 450 ms is preferable.

Intramuscular depot preparations of first and second generation antipsychotics assist with improved treatment adherence (see Table 13.12).

Mood stabilisers

This includes lithium.

Anticonvulsants

These include sodium valproate, carbamazepine and lamotrigine.

Hypnotics

These include benzodiazepines: flunitrazepam, nitrazepam and temazepam (see Tables 13.13 and 13.4).

Other psychotropic medications

Pregnancy and breastfeeding

General comments

image No psychotropic medication can be guaranteed safe in pregnancy or during lactation, but relative risks can be defined to some degree (Box 13.13). The fundamental principle is to carefully balance the risks of the patient’s illness against the known risks of the treatment. However, excessive concern over adverse effects upon the fetus can sometimes leave mother and baby at significant risk and the decision about medication should be made carefully. Specialist assistance may be particularly useful.

Other biological treatments

Electroconvulsive therapy (ECT)

References and further reading

Aszalos A. Drug–drug interactions affected by the transporter protein, P-glycoprotein (ABCB1, MDR1) II. Clinical Aspects Drug Discovery Today. 2007:838-843. 12(19/20)

Castle D., Copolov D., Wykes T., Mueser K. Pharmacological and psychosocial treatments in schizophrenia. London: Informa UK; 2008.

Dowden J., Allardice J., Ames D., et al. Therapeutic guidelines: psychotropic version. Melbourne: Therapeutic Guidelines; 2008.

Janicak P., Davis J., Preskorn S., et al. Principles and practice of psychopharmacotherapy. Philadelphia: Lippincott Williams and Wilkins; 2006.

Menon S. Psychotropic medication during pregnancy and lactation. Archives of Gynecology and Obstetrics. 2008;277(1):1-13.

Rosenbaum J., Arana G., Hyman S., et al. Handbook of psychiatric drug therapy. Philadelphia: Lippincott Williams and Wilkins; 2005.

Schatzberg A., Nemeroff C. Essentials of clinical psychopharmacology. Washington DC: American Psychiatric Publishing; 2006.

Stahl S. Essential psychopharmacology. The prescriber’s guide. Cambridge: Cambridge University Press; 2005.

Tiller J., Lyndon R., editors. Electroconvulsive therapy. An Australasian guide. Melbourne: Australian Postgraduate Medicine, 2003.

Weiner R., Fink M., Hammersley D., et al. The practice of electroconvulsive therapy: recommendations for treatment, training and privileging. In Task Force Report of the American Psychiatric Association. Washington DC: American Psychiatric Association; 1990.

Wiznen P., Op Den Buijsch R., Drent M., et al. the prevalence and clinical relevance of cytochrome P450 polymorphisms. Alimentary Pharmacology and Therapeutics. 2007;26(Suppl 2):211-219. Review article