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Pharmacokinetics and Pharmacodynamics

As advanced practice nurses it is important that we understand pharmacokinetics and pharmacodynamics when caring for our patients. There are many factors to consider when prescribing medications because not one person is the same. Pharmacokinetics is the study of drug movement throughout the body, and has four basic processes: absorption, distribution, metabolism, and excretion (Rosenthal, 2021, p 13). Pharmacodynamics is the study of the biochemical and physiologic effects of drugs on the body and the molecular mechanisms by which those effects are produced (Rosenthal, 2021, p 22). In my experience I can not pinpoint one specific scenario in which pharmacokinetic and pharmacodynamic factors altered an anticipated response to a drug but multiple scenarios.

I can think of things to consider when prescribing medications like their medical history, medications they take, whether prescribed or over the counter, compliance, genetics, age, race, and family history. Other things to consider is the patients liver function and renal function, which affect the drugs absorption and excretion rate. One class of drugs that comes to mind are chemotherapy medications. Not everyone has the same outcome with chemotherapeutic agents when treating cancers. Another class of medications are psychotropic medications, some psych medications work better for others and do not work at all for some. And one of the most common ailments hypertension, antihypertensive drugs that work for some and not others. Immunosuppressed, CHF, renal failure, liver failure patients and patients with multiple ailments on multiple medications, all have to be prescribed medications carefully because of possible polypharmacy and drug interactions. A-fib patients prescribed coumadin needing their PT/INR checked to make sure their coumadin is therapeutic, educating them not to eat green leafy vegetables or drink grapefruit juice.

A patient’s genetics may play a role in whether the medication prescribed is going to work or not. It became evident in the 60’s that therapeutic doses of psychotropic medications vary markedly from person to person, with some patients failing to respond despite treatment with high dosages (Lerer, 2004, p158). Among many factors that may influence the drug response, hepatic function, and drug metabolism ae of great importance for nearly all medications used in psychiatry and even more importantly patient compliance with medication use (Lerer, 2004, p158). The CYP2D6 genetic polymorphism was discovered in late 1970’s and represents one of the most intensively studied monogenic variations in drug metabolism, approx. 7% of Caucasians are poor metabolizers of CYP2D6 substrates while the rest are considered as extensive metabolizers (Lerer, 2004, p160). CYP2D6 enzyme has particular significance for clinical psychiatry as it is involved in clearance of many psychotropic drugs such as tricyclic, antidepressants, selective serotonin reuptake inhibitors, classical antipsychotics, some atypical antipsychotics, drugs of abuse and codeine (Lerer, 2004, p160). In patients who rapid metabolize and have multiple copies of CYP2D6, conventional dose titration may cause delays in therapeutic response and lead to prolonged inpatient hospitalization. It is important to know our patient’s, do lab work according to the drugs being prescribed and refer our patients to specialty physicians if necessary.

                Another consideration is gender when prescribing medications. Research shows that women have different absorption due to lower gastric pH and longer gastrointestinal transit, lower P-glycoprotein expression, different body fat percentage than men, and that estrogen in women can affect drug metabolizing enzymes (Pierzchlinska, 2018, p 117). All of the topics discussed are why it is so important to develop an individualized care plan for each patient to provide the best care possible. The care plan should include goals of therapy, interventions, and follow up evaluation of treatment outcomes. Well rounded individualized treatment plans specific for each patient allow patients to establish trusting relationships with their providers.

Reference

Lerer, B. (2004). Pharmacogenetics of psychotropic drugs. (electronic source). Cambridge University Press.

Pierzchlinska, A., & Bialecka, M. (2018). An introduction to gender differences in neurological treatment. Aktualnosci Neurologiczne, 18(3), 117-122. https://doi-org.ezp.waldenulibrary.org/10.15557/AN.2018.0016

Rosenthal, L.D., & Burchum, J.R. (2021). Lehne’s pharmacotherapeutics for advanced practice nurses and physician assistants (2nd ed.) St. Louis, MO: Elsevier