Tuesday, November 10, 2009

The Good Buffer

By Scott Rudge

“A Good Buffer” has a number of connotations in biochemistry and biochemical engineering. A “good buffer” would be one that has good buffering capacity at the desired pH. The best buffering capacity is at the pK of the buffer of course, although it seems buffer salts are rarely used at their pK.

Second, a good buffer would be one matched to the application. Or maybe that’s first. For example, the buffering ion in an ion exchange chromatography step should be the same charge as the resin (so as not to bind and take up resin capacity). For example, phosphate ion (negative) is a good choice for cation exchange resins (also negatively charged) like S and CM resins.

Another meaning of a “Good” buffer is a buffer described by Dr. Norman Good and colleagues in 1966 (N. E. Good, G. D. Winget, W. Winter, T. N. Connolly, S. Izawa and R. M. M. Singh (1966). "Hydrogen Ion Buffers for Biological Research". Biochemistry 5 (2): 467–477.). These twelve buffers have pK’s spanning the range 6.15 to 8.35, and are a mixture of organic acids, organic bases and zwitterions (having both an acidic and basic site). All twelve of Good’s buffers have pK’s that are fairly strongly temperature dependent, meaning that, in addition to the temperature correction required for the activity of hydrogen ion, there is an actual shift in pH that is temperature dependent. So, while a buffer can be matched to the desired pH approximately every 0.2 pH units across pH 7 ± 1, the buffers are expensive and not entirely suited to manufacturing applications.

In our view, a good buffer is one that is well understood and is designed for its intended purpose. To be designed for its intended purpose, it should be well matched to provide adequate buffering capacity at the desired pH and desired temperature. As shown in the figure, the buffering capacity of a buffer with a pK of 8 is nearly exhausted below pH 7 and above pH 9.

It’s easy to overshoot the desired pH at these “extremes”, but just such a mismatch between buffering ion and desired pH is often specified. Furthermore, buffers are frequently made by titrating to the desired pH from the pK of the base or the acid. This leads to batch to batch variation in the amount of titrant used, because of overshooting and retracing. In addition, the temperature dependence of the pK is not taken into account when specifying the temperature of the buffer. Tris has a pK of 8.06 at 20°C, so a Tris buffer used at pH 7.0 is already not a good idea at 20°C. The pK of Tris changes by -0.03 pH units for every 1°C in positive temperature change. So if the temperature specified for the pH 7.0 buffer is 5°C, the pK will have shifted to 8.51. Tris has 3% of its buffering capacity available at pH 7.0, 5°C, it’s not well matched at all.

A good buffer will have a known mass transfer rate in water, so that its mixing time can be predicted. Precise amounts of buffering acid or base and cosalt are added to give the exact pH required at the exact temperature specified. This actually reduces our reliance on measurements like pH and conductivity that can be inexact. Good buffers can be made with much more precision than ± 0.2 pH units and ± 10% of nominal conductivity, and when you start to make buffers this way, you will rely more on your balance and understanding of appropriate storage conditions for your raw materials, than making adjustments in the field with titrants, time consuming mixing and guessing whether variation in conductivity is going to upset the process.

Friday, November 6, 2009

Enzyme induction…done pharmacodynamically

By Ray Nims

Pharmacodynamics is the study of a specific effect of a drug as related to drug concentration at the putative active-site for that effect. Pharmacodynamics is sometimes used to model quantitatively the effect of a drug over time as drug concentration at the active-site rises and falls. Another type of pharmacodynamic study entails exposing the animal or in vitro system to graded doses of a drug and monitoring the effect associated with each active-site concentration. From the latter type of study, one is able to estimate both potency for the effect (given in terms of the active-site drug concentration at the half-maximal effect for that drug, or EC50) and its efficacy (given in terms of percentage of maximal response compared to other drugs causing the same effect through the same mechanism). In receptor theory, EC50 is considered to reflect the affinity of the drug for a receptor, while efficacy is a measure of the bound drug’s ability to cause the specific response.

The induction of drug-metabolizing enzymes, such as the cytochromes P450, may be considered to represent an effect of a drug or xenobiotic. It is common for investigators to measure such induction at one or a few dose levels and to compare the resulting enzyme induction with that of a prototype inducer. These comparisons are sometimes described in terms of the test xenobiotic causing “strong” (“potent”) or “weak” induction in comparison with the prototype inducer. As already pointed out quite elegantly by D. A. Smith and coworkers (Letter to the Editor: The Time to Move Cytochrome P450 Induction into Mainstream Pharmacology is Long Overdue. Drug Metab. Dispos. 35:697-698, 2007; http://dmd.aspetjournals.org/cgi/content/full/35/4/697), such statements are both misleading and inaccurate. As with any drug effect, enzyme induction must be described in terms of both potency and efficacy. It is possible for an inducer to be very potent but to display little efficacy. In fact, a xenobiotic having high potency and little or no induction efficacy might represent a competitive inhibitor for this effect. In contrast, there may be inducers which are very effective, but not very potent.

It is possible for efficacy and potency for enzyme induction to be estimated on the basis of studies using intact animals, provided that certain assumptions are made (e.g., that total plasma drug concentration is a suitable proxy for drug concentration at the induction active site, which cannot be sampled directly). An example of such a study is that of R.W. Nims and coworkers (Comparative Pharmacodynamics of Hepatic Cytochrome P450 2B Induction by 5,5-Diphenyl- and 5,5-Diethyl-substituted Barbiturates and Hydantoins in the Male F344/NCr Rat. J. Pharmacol. Exp. Therap. 270: 348-355, 1994; http://jpet.aspetjournals.org/cgi/content/abstract/270/1/348). A more straightforward approach is offered through in vitro enzyme induction studies, in which enzyme induction can be related to drug concentration in the culture medium (e.g., Kocarek and coworkers: Differentiated Induction of Cytochrome P450b/e and P450p mRNAs by Dose of Phenobarbital in Primary Cultures of Adult Rat Hepatocytes. Mol. Pharmacol. 38:440-444, 1990; http://molpharm.aspetjournals.org/cgi/content/abstract/38/4/440).

Measurement of the induction of the cytochromes P450 and other drug-metabolizing enzymes following drug treatment in animals and humans is an important aspect of drug characterization. The studies should be performed and reported in a manner consistent with other drug effects, that is, in a manner consistent with the principles of pharmacology.