Metabolites and Drug response - Clinical practice

14 important questions on Metabolites and Drug response - Clinical practice

What is empirical dosing?

You give a dose this will result in a certain concentration and this will give a certain effect. The dose is the same in every individual, the concentration can vary a bit but as a result the effect(s) will vary more.

What is pharmacodynamic dosing?

You give a dose to patients that is individualized for every person. This will result in a concentration and that will give a certain effect. In this case the dose varies the most, the concentration varies a bit but the effect(s) will be the same in every single person.

What is pharmacokinetic dose adaptation?

Again, you give a dose what results into a concentration what will give an effect(s). This time the dose can vary a little bit and also the effect, but the concentration is the same in every single person.
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What do you do when you need to give an advice about the risk of possible toxicities?

You carry out a risk assesment
- You ask the dose
- You look up the Vd
- You ask for the persons' body weight
- You calculate the cmax by using the formula cmax = dose / vd. (for the vd in this case you have to multiply it by the persons'body weight.
- You think about what the risks are of the drug.

What do you do when a male patient (70 kg, 54 years ans serum creat 80 micromol/L) must be treated with tobramycin? The physician starts with 300 mg once daily at 12:00. He measured the plasma concentrations at 13:00 and 21:00.

- You need to calculate the Vd and elimination half life with V = Dose/ C
-  Assume that the concentration at 13:00 is the C(0). Then you can calculate the Vd.
-   From the 2 data points you can get the t1/2.

Whether you should continue with a next dose of 300 mg or not is dependent of th PD and the therapeutic targets.  For Tobramycine for example the cmax/ MIC is 8-10.

What is the MIC of tobramycin?

2 mg/L and for an optimal treatment the cmax/ MIC must be somewhere between 8-10. (or at least higher).

For a recovery of tobramycin toxicity the Cmin must be lower than 0,5 mg/L. To reach this at least 5 hours are needed

What are the required data for the "a priory" dose?

- PK population parameters: mean (Vd, Cl, K12, K21), standard deviation
- Target value for relevant PK/PD determinant: Peak/through (aminoglycosides), AUC (vancomycin, fluorchinolones), T above MIC (beta-lactam antibiotics)

What is the TDM scheme based on Bayes principles?

You have patient data. This data can be used with algorithms to get "a priory" dose and this results into an serum concentration and a result. The patient data can also be used for PK model and a target. This results into a PK dosing and also into a serum concentration inclusive a result. From this serum concentration + result one can go back to the PK model, target.

So, what is the practical protocol for tobramycin TDM?

- calculate "a priori" dose based on BW.
- Draw peak serum level 0,5 hrs after first dose
- Draw second level 1.44 x average t1/2 (6-12 hours) after first dose.
- Analyse the samples, calculate individual PK parameters
- Calculate next dose and dosing interval
- Repeat sampling.

What is different about ICU patients?

They have an increase of Vd for antibiotics. Cl can continuously change. So the drug absorption, distribution, metabolism and excretion can change in those patients.

How is the distribution in ICU patients changed?

- increased total body water and edema
- decreased albumin concentration
- increased alpha-1-acid glycoprotein
-  alterations of blood pH
- septic shock

Cmax is reduced in those patients, CL is unchanged and T1/2 is increased. Also the time to steady-state is increased. This means that there is a decreased efficacy of concentration dependent drugs (aminoglycosides)

What if a decreased albumin level occurs?

Due to an increased free concentration elimination will increase. The overall effect is a reduction of the total concentration without change in free concentration. In this case it is better to adjust the dose based on a measurement of the free concentration.

What happens if AAG (alpha-1- acid glycoprotein increase?

the Vd will reduce this result in less free drug (e.g lidocaine). This means that increased blood AAG concentrations are paired to an increase blood lidocaine concentration but to a decreased effect.

How is the drug excretion in ICU patients?

- There is almost no active secretion, so the renal excretion of drugs parallels the creatinine clearance.
- ICU patients often have a renal disfunction.
- If a ICU patient has a good renal function an augmented clearance can occur due to release of inflammatory cytokines or use of e.g. norepinephrine (vasopressive drug).

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