Phytochemicals, pharmacogenetics and pharmacokinetics

When one appreciates the extensive range on interactions that exist between phytochemicals essentially entering our bodies through our diet, and the various processes involved in host protection, it becomes very clear that dietary modifications of pharmacokinetic process should be a relative given in our understanding of drug ADME. In fact it should be a central theme in our understanding of interindividual and interpopulational variability in drug behaviour, rather than just being accorded the occasional consideration as a determinant of drug behaviour.

As a corollary, pharmacogenetics cannot hope to fully explain the pharmacokinetic behaviour of any drug in an individual. Environmental chemicals, largely phytochemicals, modify pharmacokinetic processes according to the genetic constitution of the individual, while the genetic makeup of the individual can only be fully expressed in response to environmental chemical effects.

Which leads us to automatically consider what the epigenetic mechanisms might be, in shaping the PK environment of the individual. Perhaps these might be more important than the occasional loss/gain of function variants that we find in the population.

Polyphenols - another type of phytochemical

Another probably more significant group of phytochemicals are the polyphenols. These are molecules of various sizes but which have more than one phenolic unit in their structure. Examples of polyphenols are flavonoids, lignins and tannins.

One of the oldest functions of polyphenols might be protection against UV damage. But polyphenols tend to have varied and complex biological roles. Some of these roles include antioxidation, cell signalling and insect/herbivore signalling.

The non-flavonoid polyphenol, curcumin, for example, is principle member of a family of co

ngeners found in turmeric.

Turmeric itself is a rhizome and a relative of the ginger. The Chinese call it the 'yellow ginger', and the Malays call it 'kunyit'. It's biological activity is recognized in in many cultures and is listed in many traditional pharmacopoieas. It is best known however for the yellow flavouring used in many curries.

Curcumin is however, actually, poorly absorbed when taken orally. The reason has been attributed to poor absorption and rapid metabolism and elimination. It has been shown to induce apoptosis of cancer cells, and thus thought to have anti-cancer properties in the colon. Some clinical studies have also shown that curcumin taken in gram amounts over a period of time can inhibit CYP1A2 while enhancing CYP2A6. Apparently, when given in these large amounts, significant absorption occurs to enable enzyme inhibition. In vitro work suggest inhibition also of CYP2B6 and CYP3A4. Curcumin is otherwise quite harmless even in large doses and therefore appear to serve only the purpose of discouraging eating of the raw rhizome. Although quite flavourful when cooked with other spices, the raw turmeric root is quite unpalatable due to the bitter and pungent taste of curcuminoids.

Similar to the alkaloids, the polyphenols interact extensively with drug metabolizing enzymes and membrane transporters. These interactions are not limited to direct interactions with the proteins, but are also mediated through interaction with regulatory processes of the various enzyme and transporter genes.

Plant alkaloids and chemodefense

Although many alkaloids have toxicity which is immediate and topical so as to discourage predation, many alkaloids do get absorbed into the predator's body and can therefore produce pharmacological and toxicological effects beyond the point of exposure. Some of these effects are extreme and may cause severe reactions in the predator, again discouraging predation.

Animals learn to stay away from such plants. Alternatively, they develop protective mechanisms against the toxicological effects of the alkaloids. Apart from the biological membranes which provide an initial protective barrier against insoluble and hydrophilic chemicals, many organisms also have evolved protective mechanisms such as the cytochrome P450 enzyme systems and the efflux transport proteins to detoxify and repel the more permeable alkaloids which may be able to escape past the biological membranes. In response, plants, over time, evolve even more complex chemicals to overcome animal defense mechanisms. This plant-animal arms-race create the complex environment which now can be seen to determine pharmacokinetic behaviour of the the drugs we use.

Drug metabolism and drug transport must therefore be seen as component parts of an integrated process to protect animals from the toxicity of plant alkaloids.

Read this interesting account of the mustard oil bomb.

A well known groups of alkaloids are the methylxanthines.

Caffeine: R₁ = R₂ = R₃ = CH₃

Theobromine: R₁ = H, R₂ = R₃ = CH₃

Theophylline: R₁ = R₂ = CH₃, R₃ = H

The three main members are caffeine (1,3,7-trimethyxanthine), theobromine (3,7-dimethylxanthine) and theophylline (1,3-diethyxanthine). Caffeine is found in tea and coffee, while theobromine is the main methylxanthine found chocolate. The methylxanthines are phospohodiesterase inhibitors.

The metabolism of caffeine is shown below. Caffeine has been use as a probe substrate to develop metabolic ratios for CYP1A2 and N-acetytransferase 2.

Plant alkaloids

Alkaloids at chemicals with a nitrogen in the structure. Most are basic bit this not necessarily so as some may be neutral or weak acids. By and large the alkaloids are cyclic structures with the nitrogen as part of the ring. Exo-cyclic nitrogenous bases are just called amines.

Alkaloids can be found both in plants and animals, but for the most part we refer to plant alkaloids. For a long time, plant alkaloids were though to be incidental metabolic dead-ends which accumulated in certain plant parts, but that line of thinking has changed, and now it is thought that plant alkaloids are part of the plant defense mechanisms against insects and animals.

Alkaloids are often biologically active and tend to be bitter in taste, reflecting their role in discouraging insect and animal attacks. Their effects are often topical and immediate, and it is not necessary for them to be absorbed into the body. Many therefore are bitter tasting, emetic or intestinally toxic. They may also be directly tissue toxic, for example, producing contact dermatitis when applied to the skin.

Alkaloids are often not produced in isolation in the plant and exist as part of a family of congeners. They are found in growing parts of plants, young shoots and leaves, barks and roots. These basket of chemicals produce a variety biological effects, targeting a wide range of biological systems. Because of their versatility in interacting with biological systems, many alkaloids can produce 'beneficial' effects. For the most part, I believe these are incidental. It is therefore a

fallacy, promulgated by new age naturopaths and companies selling natural remedies, that chemicals from plants are naturally 'health giving' and safe.

The Han Chinese - fiction or biological reality?

The Chinese people have been called various names. Once we were called Orientals; the word 'orient' referring to the East. The original reference to Orientals by the west was with respect to people from the middle east or asia minor- regions East of Europe. In time, the definition shifted eastwards and became more associated with East Asia. Clearly the term has no meaning today and should be completely dropped from the scientific literature. Another term, 'Asians' was sometimes used, although in the United Kingdom, this has come to refer to people from the Indian subcontinent. The term East Asians attempts to shift the definition towards people from China, Korea and Japan. For the most part, Chinese were just referred to as Chinese.

More recently however, another term 'Han Chinese' has emerged. The implication appears to be that the concept of Chinese was inadequate and that somehow a 'Han' preface was needed to distinguish Han Chinese from non-Han Chinese. I suspect this has arisen as a result of more scientific work from China, where it was necessary to distinguish the main Han ethnic group from the other 56 ethnic groups which were not 'Han'. The following chart shows the increasing use of the term 'Han Chinese' in the literature beginning from about 1980 and rising steeply over the last 10 years.

In Singapore we don't really care to be Han Chinese, other than using the term to be consistent with 1 billion other people who want to use the term. For us, we are just plain and simple Chinese.

The term Han Chinese actually takes reference to a wish to be associated with the legendary 1000 year Han dynasty which ended in 220CE. I think this is more important to the mainland Chinese, more of the northern sorts. Interestingly for many of us more southern type Chinese, of the Min-nan dialects, the term 'Tang people' '唐人' is more frequently used. This is in reference to another wonderful period of Chinese history called the Tang dynasty (618-904).

So there isn't a whole lot of consistency with respect to how Chinese people want to refer to themselves. Tracing an ancestry back to a dynastic period is not an ethnically reasonable proposition since there can be no assumption that people from that dynasty were necessarily ethnically homogeneous.

It is somewhat of a wishful thinking that the Chinese people, whether Han or Tang, are a biologically homogeneous bunch of people. For large stretches of Chinese history, the country had to deal with more belligerent northern neighbours, and had been conquered by Mongols and Manchus from the North. These people, clearly of a different ethnicity from the indigenous Chinese, not only tended to displace the populations southwards and also widely assimilated with the resident Chinese. The more southern Chinese didn't have the benefits of the Great Wall so there was really nothing to keep them from mingling with their more southern non-Han neighbours.

As a result of these admixtures, there is a north-south cline in East Asian populations. In China itself, there would appear at the very least, among the "Han Chinese", to be a distinction between northern and southern Chinese. The watershed for these two groups is not a clear one, but appear to be somewhere in the vicinity of the Yangtze or Chang Jiang river divide.

It is important to be aware of the existence of this cline because it affects the way we look at "East Asian" population data. While for the most part, Chinese data tend to cluster quite closely together (which is not surprising), we need to be careful about too readily extrapolating northern data to a southern Chinese population, and vice versa. In many situations, northern Chinese data seem to be more in alignment with data from Japan and Korea, while southern Chinese data aligns with Hong Kong, Taiwan and Singapore.

Singapore, being the most southern of the major 'Han Chinese' populations, may have some unique features potentially because of some degree of genetic admixtures with the resident austronesian populations in south east Asia.

Optimization? What's that?

Optimization refers to the situation where you can adjust the inputs into a system according to output functions, and eventually arrive at the best solution. In therapeutic terms, it is the process of calibrating the dosage of a drug according to the clinical response so that the best dosage for the patient can be arrived at depending on the therapeutic targets and the patient's individualized response.

This is really no different from the engineering concept of a control system.

When there is an input into a particular process, and no feedback is received about the outcome....this is called an 'open loop control system'. This is probably the least ideal of all control systems. It fundamentally assumes you know everything there is to know and that the decision taken about the initial input is already adequate. This happens in therapeutics when you have fixed dose regiments, and there is no feedback about the outcome. Consider the situation in most cancer chemotherapeutic regiments. Dosage regiments are pretty much determined at the outset, and the only feedback received is if the patient has obvious toxicity which requires cessation of therapy, i.e. switch off the system! This is essentially the pharmacogenomic approach towards 'personalized medicine'.

In other scenarios, there is possibility for the operator to make adjustments to the original input based on feedback received, but this takes place independent of the original model which determined the input. This is called an "open-loop feedback control system'. Most therapeutic scenarios are of this sort, where an initial decision about a starting dosage regiment is made based on starting knowledge and assumptions about the patient. Subsequently minor adjustments to the dosage regiment can be made by the physician depending on feedback received about the patient's clinical drug response. Where there is poor ability to receive feedback about clinical drug response, the system starts to flounder and approximates the simple open-loop control system.

The open-loop feedback control system can operate with varying degrees of sophistication. It can be empirical, where the response to feedback received is relatively intuitive and based on clinical judgement. Or it can be highly deterministic where the response is determined by precise mathematical (PK or PK-PD) models.

A more sophisticated model takes into consideration the uncertainty in the system. This is called a stochastic approach.

The ideal system is that of a closed loop system where the original models determining the original input is linked to, and continually modified by new feedback received. The following diagram represents a closed loop control system with warfarin as an example.

Modified from Applied pharmacokinetics & pharmacodynamics: principles of therapeutic drug monitoring. 1992 Michael E. Burton et al.

At top-right there is a population PK-PD model of warfarin. This represents what is known about how warfarin behaves in the population in which the patient exists. Together with the clinical model at top-left, of what the desired or target INR response is, a decision about the starting dosage regiment can be made.

Subsequently, feedback is regularly received about warfarin pharmacokinetics (bottom right) and INR response (bottom left). These feedback into the original models at top right and left, and continually adjust the model so that decisions are continually taken about how the dosage regiment can be adjusted.

This is optimization... and represents what personalized medicine ought to be.

Can it be done for all drugs? Yes, it can. But it requires that there are good population PK-PD models for the drug, and good biomarkers of response that can be used as feedback. It requires resources and effort.

Above all, it will require that physicians be prepared to put in the extra effort to optimize their therapy according the the patient's real requirements.