Zero order kinetics is unchanged elimination speed all the time with no dependance of drug concentration, first order kinetics is elimination dependant on the concentration of the drug. What is 2nd (3rd etc) order kinetics in plain terms? I am lost in differential equations and need a plain intuitive explanation ...
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N&PD Moderators: Skorpio | thegreenhand
Explain me 2nd and higher order kinetics in plain terms, please
- Thread starter Renald
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There's no need to solve any kind of differential equations to understand the concept, it is fairly simple. Basically, if you start thinking from how the process works, then you can get an analogy from the conditions for a successful "action". For example, radioactive decay comes from within the atom, you cannot influence the rate at which atoms decay chemically (although you can by bombarding the nuclei, of course). So, each atom's decay depends only on the atom itself, not its surroundings. Therefore, the kinetic equation for that would be v=k*c (where v is for reaction speed, dc/dt, k is a random constant, and c is the concentration of the substance) - that means that the fewer atoms there is, the slower the overall process, or vice versa.
A simple way to imagine a second-order process is a chemical reaction in which the process occurs when a molecule of substance A collides with a molecule of substance B. So, it's dependent on the concentration/whatever of both A and B. Therefore, the kinetic equation for that would be v=k*[A]* ([A] is concentration of substance A). Now, situations where 3 molecules collide all at the same time in a way that's required for a chemical reaction to occur are rare, but the basic principle of orders, as I would put it, describes how heavily the process is dependent on concentration(s) of substance(s). So a third-order process could be v=k*[A]*^2, which means that it's first-order regarding substance A and second-order for B.
How this all applies in regard to drug elimination is a bit more complex, because biological systems are complex. It's easy and makes sense for zero and first-order types of situations. Zero-order is analogous to, for example, certain catalytic processes where the speed of reaction is determined by the surface of the catalyst rather than the concentration/partial pressure of the reagent - in drug elimination it's just that the catalyst is an enzyme. First-order elimination, which is the norm for drug elimination, is akin to radioactive decay. Second and higher orders are ought to be more complex because there are other processes or conditions which deviate it from following first-order kinetics, depends on the circumstances and the drug.
I would like to stress that the use of kinetic models is only trying to describe how the system works, it is not an inherent property of a certain substance.
A simple way to imagine a second-order process is a chemical reaction in which the process occurs when a molecule of substance A collides with a molecule of substance B. So, it's dependent on the concentration/whatever of both A and B. Therefore, the kinetic equation for that would be v=k*[A]* ([A] is concentration of substance A). Now, situations where 3 molecules collide all at the same time in a way that's required for a chemical reaction to occur are rare, but the basic principle of orders, as I would put it, describes how heavily the process is dependent on concentration(s) of substance(s). So a third-order process could be v=k*[A]*^2, which means that it's first-order regarding substance A and second-order for B.
How this all applies in regard to drug elimination is a bit more complex, because biological systems are complex. It's easy and makes sense for zero and first-order types of situations. Zero-order is analogous to, for example, certain catalytic processes where the speed of reaction is determined by the surface of the catalyst rather than the concentration/partial pressure of the reagent - in drug elimination it's just that the catalyst is an enzyme. First-order elimination, which is the norm for drug elimination, is akin to radioactive decay. Second and higher orders are ought to be more complex because there are other processes or conditions which deviate it from following first-order kinetics, depends on the circumstances and the drug.
I would like to stress that the use of kinetic models is only trying to describe how the system works, it is not an inherent property of a certain substance.