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Potensi obat

Apa yang dimaksud potensi? Sering kita mendengar, obat X lebih poten dibanding Y, atau obat glibenklamid lebih poten dibanding tolbutamid, karena glibenklamid cukup 5 mg saja untuk menghasilkan efek, bandingkan dengan tolbutamid (generasi pertama) butuh  500 mg. Ini artinya sulfonilurea generasi II umumnya potensi hipoglikemiknya 100x lebih besar dari generasi I. 

Jika kita lihat kurva hubungan konsentrasi-efek, sumbu x adalah konsentrasi yang menunjukkan potensi suatu obat. Konsentrasi dan dosis, apakah sama? Apakah bisa saling dipertukarkan? Jawabannya tidak. Konsentrasi lebih dikaitkan dengan konsentrasi obat di dalam plasma dalam kondisi sistem yang terisolasi secara in vitro dan untuk menghindari faktor variabel farmakokinetika yang rumit. Sedangkan dosis adalah dalam sistem organisme utuh dalam menghasilkan efek.

konsentras-efek curva

Gambar. vContoh kurva konsentrasi-respon pada sistem in vitro. Log konsentrasi-respon biasanya sogmoid, namun bagian antara 10% dan 90% dari respon maksimal mendekati garis lurus dan biasanya terjadi lebih dari dua kali lipat konsentrasi. Kurva A adalah full agonist dengan EC50 sekitar 3×10(-8)M (intercept C), kurva B adalah partial agonist pada reseptor yang sama.

Potensi jelas mempengaruhi dosis obat, ya kan? Namun, potensi sendiri relatif tidak penting dalam klinis sepanjang dosis yang diperlukan dapat diberikan dengan nyaman dan tidak menimbulkan toksisitas. Ambil contoh parasetamol butuh dosis 500 mg untuk bisa berefek, namun obat ini tetap dipakai walau ada analgesik lain yang potensinya lebih besar (misal ada obat yang cukup butuh 2 mg saja untuk berefek). Jadi, tidak ada pembenaran terhadap adanya pandangan bahwa obat yang lebih kuat merupakan senyawa terapetik yang lebih baik.

Namun, jika obat diberikan melalui absorpsi transdermal, maka diperlukan obat yang sangat poten, karena kapasitas kulit dalam mengabsorpsi obat terbatas.

Agonist

Suatu obat atau senyawa yang berikatan dengan reseptor dan mengaktifkannya, menghasilkan respon farmakologis (kontraksi, relaksasi, sekresi, aktivasi enzim, dll).

Antagonist

Suatu obat atau senyawa yang melemahkan efek agonis. Macamnya bisa kompetitif, contoh mengikat reversibel ke daerah reseptor yang sama dengan agonis, menempati situs yang sama namun tanpa mengaktifkan mekanisme efektor. Efek dari antagonis kompetitif dapat diatasi dengan meningkatkan konsentrasi agonis, sehingga menggeser kesetimbangan dan meningkatkan proporsi pendudukan agonis pada reseptor.

Jenis antagonis lain yaitu antagonis unsurmountable, di mana agonis tidak sepenuhnya dapat mengatasi hambatan tersebut setelah diberikan. Antagonis unsurmountable mengikat secara kovalen ke situs pengikatan agonis (antagonis ireversibel kompetitif), dalam hal ini ada periode sebelum bentuk ikatan kovalen bersaing dengan ligan dapat mencegah penghambatan.

Jenis lain dari antagonis unsurmountable beraksi secara allosterik di lokasi yang berbeda pada reseptor atau saluran ion yang terkait. Lihat Jenkinson ( 1991) dan Rang dan Dale ( 1991) untuk lebih detail. Juga lihat antagonisme fungsional.

EC50

The molar concentration of an agonist which produces 50% of the maximum possible response for that agonist. Other percentage values (EC25, EC40, etc) are sometimes used.

Efficacy
A term introduced by Stephenson (1956) to describe the way in which agonists vary in the response they produce even when they occupy the same number of receptors. High-efficacy agonists can produce their maximal response while occupying a relatively low proportion of receptors; agonists of lower efficacy cannot activate the receptors to the same degree and may not be able to produce the same maximal response even when they occupy the entire receptor population, thereby behaving as partial agonists.

Functional antagonism
(or physiological antagonism). Reversal of the effects of a drug by an agent which, rather than acting at the same receptor, causes a response in the tissue or animal which opposes that induced by the drug. Examples include agents which have opposing effects on an intracellular second messenger, or, in an animal, on blood pressure. A functional antagonist can sometimes produce responses which closely mimic those of the pharmacological kind.

IC50

Where an agonist causes an inhibitory response, the IC50 is the molar concentration which produces 50% of its maximum possible inhibition.

Intrinsic activity
A term devised by Ariens in 1954 which attempted to describe the mathematical relationship between receptor occupancy and tissue response. It has now largely been replaced by efficacy, because the definition of intrinsic activity means that it varies for a particular agonist between different tissues, but efficacy, in theory, does not. However, intrinsic activity is now widely used as an empirical measure of the maximal response to a test agonist as a fraction of that to a full agonist of the same pharmacological class.

Inverse agonist
A drug which produces an effect opposite to that of an agonist, yet acts at the same receptor. The best established examples act at the benzodiazepine receptor (see Schofield, 1989). Such compounds have also been described as negative antagonists, or as having negative efficacy.

KA
The dissociation equilibrium constant for an agonist. It is difficult to determine experimentally, since it does not necessarily equal the concentration which produces half-maximal response nor the concentration which occupies 50% of the receptors (see efficacy). It may be measured by Furchgott analysis; alternatively, if assay conditions are identical, it may equal the Ki value determined in a binding assay. The reciprocal is called the affinity constant or association constant. Do not confuse with the physicochemical use of the same symbol. For more detailed information, see Jenkinson (1989)

Occupancy
The proportion of receptors to which a drug is bound. It may be calculated from the Hill-Langmuir adsorption isotherm:

                   [D]      
 Occupancy  =  ___________

                K  +  [D]

where K is the dissociation constant for the drug and [D] is its concentration.

Partial agonist
An agonist which, no matter how high a concentration is applied, is unable to produce maximal activation of the receptors. In a preparation with a low receptor reserve, it is therefore unable to produce a maximal response

pD2
The negative logarithm of the EC50 or IC50 value.

Potency
A measure of the concentrations of a drug at which it is effective. A much-abused, vague term which should always be further defined. For agonists, EC50IC50KA or pD2 are usually used, whilepA2KB or pKB are used for antagonists. Other terms are used in binding studies (see section 3) which do not distinguish between agonists and antagonists. It is important to realise that the potency of an agonist does not give any information about its affinity for the receptor, because the pharmacological response is rarely directly proportional to receptor occupancy (see efficacy).

Receptor reserve
Because high-efficacy compounds need to occupy relatively few receptors to produce a maximal response, it is possible to inactivate a proportion of the receptors in a tissue (e.g. in the presence of anirreversible antagonist) without depressing the maximum of the concentration-response curve. (The curve is, however, shifted rightward along the x axis). There is said to be a receptor reserve (or, more colloquially, spare receptors) for that particular agonist in that particular tissue. There is no receptor reserve for a drug which acts as a partial agonist in the tissue. The receptor reserve may vary between tissues, depending on the number of receptors in the particular tissue and the efficiency of coupling between them and their effector mechanism. Consequently, a partial agonist in one tissue may appear to act as a full agonist in a tissue with a higher receptor reserve.

Relative efficacy
Stephenson (1956) originally proposed a numerical definition of agonist efficacy in which a pure antagonist (i.e. one totally devoid of any agonist activity) was defined as having zero efficacy, and a drug with an efficacy of 1 would, by definition, produce a maximal response at full occupancy that was 50% of the maximal response to a high-efficacy agonist. A more practical method of comparing agonist efficacy is to determine relative efficacy, i.e. to compare the ratio of the receptor occupancy at which two agonists produce the same response. There is no upper limit on the numerical value of efficacy or relative efficacy. See intrinsic efficacy.

Relative potency
The ratio of the potency of a test drug (i.e. its EC50, IC50, etc.) to that of a standard drug.

Second messenger
Intracellular substance (e.g. cyclic AMP or inositol phosphates), the concentration of which may be controlled by activation of membrane receptors and which can control further intracellular events (e.g. protein phosphorylation, neurotransmitter release or membrane polarisation).

Selectivity
Relative potency of a drug between two receptor subtypes for the same endogenous ligand. This is a relative rather than absolute term that should always be qualified (e.g. prazosin is 30-fold selective for <28>1-adrenoceptors relative to <28>2-adrenoceptors). Compare specificity.

Specificity
Relative potency of a drug between the receptors for two different endogenous ligands (e.g. sulpiride is specific for dopamine receptors when compared with 5HT receptors). Compare selectivity.

Subtype
Subtypes of receptor are those which, in a single species, are activated by the same family of endogenous ligands but exhibit sufficient differences in their pharmacological properties or molecular structure to justify being classified separately. Traditionally, subtypes have been identified using drugs which can selectively activate them or antagonise the effects of agonists with markedly different potencies (the usual rule of thumb is that there should be at least a 10-fold difference in antagonist affinity, i.e. one log unit difference in pKB value, when postulating the existence of a novel receptor subtype (Kenakin et al., 1992)). Consequently, subtypes can only be identified when pharmacological tools are available. Molecular biological techniques have now determined the amino-acid sequence of many receptor proteins, and hence the degree of homology between receptor types to be measured. However, there is no established rule which differentiates receptor subtypes simply on the basis of the number of amino-acids which differ between them. Compare species homologue.

Tachyphylaxis
A reduction in the response to an agonist while it is continuously present at the receptor, or a progressive reduction in the response upon repeated presentation of the agonist

Referensi

GlaxoWellcome

Goodman and Gilman, Farmakologi, edisi 10, (Edisi terjemahana), halaman 48

Datte and Offoumou, 2003, Involvement of nitric oxide in fading of 5-hydroxytryptamine-induced vasocontraction in rat isolated vena portae smooth muscle, J Pharm Pharmaceut Sci (www.ualberta.ca/~csps) 7(1):1-7. Download

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menebar ilmu pengetahuan

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