oral bioavailability

The oral bioavailability of Strophanthus glycosides again and again has been the subject of scientific debates from the beginning of Strophanthin therapy. Both the oral bioavailability of the "Tinctura Strophanthia ", and those of the Strophoral and those of the Strodival and other ouabain preparations were disputed.

 

The bioavailability indicates what proportion of a drug is available in the bloodstream. Whith intravenously administered drugs the bioavailability is 100 percent. The bioavailability observed following oral administration is called oral bioavailability. The absolute oral bioavailability indicates to what extent (in percent of the applied dose) the drug is absorbed after oral administration and may exert its effect in the systemic circulation.

 

The bioinformatics company PharmaInformatic has developed a database in which world-wide all published bioavailability data for drugs are included.

                             source: PharmaInformatic, http://www.pharmainformatic.com

 

The average oral bioavailability of drugs is approximately 54 percent. For about 28 percent of the drugs it is less than 30 percent. 12 percent of all drugs have a bioavailability of less than 10 percent. These values put into perspective the importance of the absolute bioavailability of a drug. For the assessment of the systemic availability of a drug preparation it is of low importance. Solely important is the question whether with a galenic formulation for oral administration, a serum concentration, can be obtained and maintained for a longer period which is necessary for a therapeutic effect. In many cases a low absolute bioavailability is sufficient for this purpose.

Several drugs with high-volume sales that are frequently used in Germany have only a moderate absolute bioavailability. The recently approved blood pressure lowering drug aliskiren has a bioavailability of 2.5 percent, dabigatranetexilate (anticoagulant) that also only recently has been approved achieves 6.5 percent, the calcium antagonist nisoldipine 5 percent. Even the classical ACE inhibitor ramipril has a modest bioavailability of only 15 percent.

To guarantee a safe therapeutic efficacy a drug has to achieve a systemic concentration that is above the "no-effect level" (NOEL) and below the "maximum tolerated dose" (MTD). A high absolute bioavailability increases the likelihood that systemic levels can be achieved above the NOEL. But it is primarily only of economic importance. The higher the absolute bioavailability, the fewer amounts of drug has to be used and thus the higher the profitability.

 

In clinical practice, daily doses of 0.25 mg ouabain administered iv have proved to be optimal. Herewith a steady-state concentration of about 0.5 ng per ml of serum is obtained [1]. An analysis of several published bioavailability data for ouabain preparations shows that this concentration can be achieved after oral administration of ouabain.

 

 

[1] Selden R, Smith TW. Ouabain pharmacokinetics in dog and man. Determination by radioimmunoassay. Circulation. 1972; 45 (6) :1176-1182.

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