Category: NUR2202C

How does ageing impact on drug dosing

Physiological changes associated with ageing can impact the appropriate dosing for many drugs. General principles to keep in mind include:

Absorption:

  • Absorption usually does not change with normal ageing.

Distribution:

  • Concentrations of water-soluble drugs are usually higher as there is less water and so a lower volume of distribution.
  • Concentrations of free or active (unbound) drug are usually higher due to lower serum proteins.

Metabolism:

  • The half-life of lipophilic drugs is usually higher due to more fat resulting in an increased volume of distribution and prolonged duration of action.
  • There is slower Phase I metabolism (e.g., oxidation, reduction and dealkylation) due to cytochrome P450 pathways resulting in higher levels of drugs dependent on these pathways for metabolism (e.g., warfarin).
  • However, Phase II reactions (e.g., conjugation, acetylation, and methylation) are usually unchanged in normal ageing.
  • There is a greater risk of drug-drug interactions in metabolism due to increased numbers of drugs for multiple medical problems.

Excretion:

  • Hepatic excretion may be impaired.
  • Renal clearance may be impaired, and serum creatinine may not be an accurate reflection of renal clearance in elderly patients due to decreased lean body mass (muscle mass).
  • Active drug metabolites can accumulate, resulting in prolonged therapeutic actions and a greater risk of adverse effects.

There is also increased susceptibility to adverse effects. Older adults are also more likely to have multiple chronic medical problems, and disease states can result in physiological changes:

  • Cardiac disease can result in impaired cardiac output resulting in impaired ADME and greater susceptibility to cardiac adverse effects.
  • Liver or kidney disease can decrease metabolism and excretion, reducing drug clearance.
  • Neurological diseases result in greater sensitivity to neurological adverse effects due to diminished neurotransmitter levels and/or impaired cerebral blood flow.

 

What is the difference between pharmacology and pharmacy?

What is the difference between pharmacology and pharmacy?

Pharmacology is the study of the sources, uses, and mechanisms of action of drugs. That is what the body does to drugs (pharmacokinetics) and what drugs do to the body (pharmacodynamics).

Pharmacy is the science or practice of the preparation, formulation,  and dispensing of medicinal drugs.

 

Aspirin for prevention of preeclampsia

Non-steroidal anti-inflammatory drugs (NSAIDs) are contraindicated in the third trimester of pregnancy because of the risk of premature closure of the ductus arteriosus. So why is aspirin used to prevent preeclampsia?

Low-dose aspirin is used to prevent preeclampsia in women at high risk of developing preeclampsia. However, NSAIDs are known to promote closure of the ductus arteriosus (see Cyclooxygenase inhibitors for closure of the ductus arteriosus) and so are contraindicated in the third trimester of pregnancy.  So why is aspirin used to prevent preeclampsia?

Preeclampsia is associated with increased platelet turnover and increases in platelet-derived thromboxane levels. Low doses of aspirin once per day are sufficient to be antiplatelet and reduce thromboxane production by the platelets. Such low doses are unlikely likely to trigger closure of the ductus arteriousus and so are relatively safe even in the third trimester of pregnancy. Thus, for the women at high risk of preeclampsia the risk-to-benefit ratio is in favour of prescribing low-dose aspirin.

Additionally, it has been reported that preeclampsia is associated with exaggerated inflammatory responses. The anti-inflammatory actions of aspirin may therefore also be beneficial in preventing preeclampsia, although the low doses used would not produce a strong anti-inflammatory effect.

There remains debate over the optimal dose and the best time to start aspirin treatment. Typically, doses between 75 mg and 162 mg/day have been used started typically before 12 weeks of gestation and certainly before 16 weeks.

Reference:

August, P & Jeyabalan, A (2019) Preeclampsia: Prevention. Lockwood, CJ & Barss, VA ed. UpToDate. Waltham, MA: UpToDate Inc. http://www.uptodate.com (Accessed on February 19, 2019).

Cyclooxygenase inhibitors for closure of the ductus arteriosus

Why is it that older non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen or indomethacin, not newer NSAIDs, such as etoricoxib, are used to promote closure of patent ductus arteriosus in preterm infants?

In the fetus, the ductus arteriosus acts as a lung bypass diverting blood from the pulmonary artery into the aorta.  After birth, the ductus arteriosus constricts and is eventually obliterated. In preterm births, the ductus arteriosus may remain patent resulting in insufficient blood flow through the pulmonary circulation and increased risk of mortality.

Prostaglandin E2 (PGE2) is a vasodilator promoting patency of the ductus arteriosus. NSAIDs inhibit the cyclooxygenase (COX) enzyme responsible for producing  PGE2. NSAIDs are therefore contraindicated in the third trimester of pregnancy as they can cause premature closure of the ductus arteriosus in utero. However, in preterm infants, NSAIDs can be valuable in enabling closure of patent ductus arteriosus (PDA).

The NSAIDs used are typically ibuprofen or indomethacin. These are older NSAIDs for which there is a longer history of experience with use in infants. Ibuprofen is generally the preferred agent as it has a lower risk of reducing gastrointestinal and renal blood flow resulting in necrotizing enterocolitis and transient renal insufficiency. The newer coxibs, such as etorixocib, are not used because there is less knowledge of their safety in infants.

Reference:

Philips III, JB (2018) Management of patent ductus ateriosus in preterm infants. Garcia-Prats JA, Fulton DR, Kim MS ed. UpToDate. Waltham, MA: UpToDate Inc. http://www.uptodate.com (Accessed on October 5, 2018).

 

 

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