Cardiac Glycosides Chemische Eigenschaften,Einsatz,Produktion Methoden
Clinical Use
Although the primary clinical use for digoxin is in the treatment of CHF, this agent also is used in
cases of atrial flutter or fibrillation and paroxysmal atrial tachycardia.
The therapeutic effects of all cardiac glycosides on the heart are qualitatively similar; however, the
glycosides largely differ in their pharmacokinetic properties. The latter are greatly influenced by the
lipophilic character of each glycoside. In general, cardiac glycosides with more lipophilic character
are absorbed faster and exhibit longer duration of action as a result of a slower urinary excretion
rate. The lipophilicity of a cardiac glycoside is measured by its partitioning between chloroform and
water mixed with methanol: The higher the concentration of the cardiac glycoside in the chloroform
phase, the higher its partition coefficient, and the more lipophilic it is. It is evident from a comparison of the coefficients
that their lipophilicity is markedly influenced by the number of sugar molecules and the number of
hydroxyl groups on the aglycone part of a given glycoside. Lanatoside C, with a partition coefficient
of 16.2, is far less lipophilic than that of acetyldigoxin (partition coefficient, 98), which structurally
differs only in lacking the terminal glucose molecule. Likewise, a comparison of digitoxin and digoxin
structures reveals that they only differ by an extra hydroxyl in digoxin at C-12. This seemingly minor
difference in their partition coefficients from 96.5 to 81.5 for digitoxin and digoxin, respectively,
results in significant differences in their pharmacokinetic behavior.The glycoside
G-strophanthin (ouabain) possesses a very low lipophilic character because of the presence of five
free hydroxyl groups on the steroid nucleus of the aglycone ouabagenin.
Nebenwirkungen
The most common and severe side effect of PDE3 inhibitors is ventricular arrhythmias, some of
which may be life-threatening. Other side effects included headaches and hypotension, which are
not uncommon for drugs that increase cAMP in cardiac and vascular tissues, with other examples
being β-agonists.
Enzyminhibitor
The mechanism of cardiac contraction involves a G-protein signal transduction pathway, which
regulates intracellular calcium concentrations. Activation of the Gs -protein involves the formation of
intracellular cAMP, which thereby increases intracellular calcium, stimulating cardiac muscle
contraction. Relaxation occurs when the released cAMP is hydrolyzed by cytosolic
cAMP-dependent PDE3, one of the phosphodiesterase isofoms. Therefore, inhibition of PDE3
increases intracellular cAMP, promoting cardiac muscle contraction but vasodilation of vascular
smooth muscle.
The overall cardiostimulatory and vasodilatory actions of PDE3 inhibitors make them suitable for the
treatment of heart failure, because vascular smooth muscle relaxation reduces ventricular wall stress
and the oxygen demands placed on the failing heart. The cardiostimulatory effects of the PDE3
inhibitors increases inotropy, which further enhances stroke volume and ejection fraction. Clinical
trials have shown that long-term therapy with PDE3 inhibitors increases mortality in heart failure
patients. Therefore, these PDE3 inhibitors are not used for the long-term, chronic therapy of CHF.
They are very useful, however, in treating acute, decompensated heart failure or temporary bouts of
decompensated chronic failure. They are not used as a monotherapy. Instead, they are used in
conjunction with other treatment modalities, such as diuretics, angiotensin-converting enzyme
inhibitors, β-blockers, or cardiac glycosides. The PDE3 inhibitors contract cardiac muscle and are
used for treating heart failure, whereas the phosphodiesterase 5 (PDE5) inhibitors are vasodilators
and are used for treating male erectile dysfunction. Note that the generic names for PDE3 inhibitors
end in “ one,” and those for the PDE5 inhibitors end in “fil.”
Cardiac Glycosides Upstream-Materialien And Downstream Produkte
Upstream-Materialien
Downstream Produkte
