Topic 6.docx

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Topic 6: Toxicokinetics - Distribution Part 1 Introduction  Give a quick and easy definition of what distribution is. o Distribution is the second phase of toxicokinetic process: this is where we define where in the body a xenobiotic will go after absorption  Explain how and why distribution is tissue-specific. o Firstly, when we say tissue-specific the idea is that tissue distribution is "perfusion-limited": different tissues will have different limits of how much blood goes there o First we talk about the highly-perfused tissues: these tissues get more blood and thus they are more vulnerable to attack by the xenobiotics which are traveling IN the blood  Liver is main biotransformation and metabolic center  Kidney as well b/c they filter and purify blood  Lungs get 100% b/c they give it oxygen  Brain gets 25% o Then there are the poorly perfused tissues, which get less blood and are thus often less vulnerable  Skin: although that may vary b/c you sweat more on a hot day and more blood goes to skin  Fat: very little active metabolism  Skeletal/connective tissues  Bone  Muscle (variable): for example, during exercise the perfusion rate would increase  OK we have given some functional reasons why tissues are either poorly or highly perfused. Now, how about another factor which can affect distribution from blood to tissues? o Internal membrane barriers affect distribution from blood to tissues: these are usually made up of cells that constitute a thin layer/lining of various body compartments o Blood-brain barrier (BBB): this is a physical barrier that interposes itself between the circulating blood and the interior of the brain tissue  You can't see this…it's not a visible envelope (that is the meningeal membrane) -- but it is a microscopic barrier constructed at the level of the blood capillaries that carry blood to the brain tissue o Blood-testis barrier (BTB): these are for blocking the spermatozoa/sperm- forming tissue from certain XB's in the bloodstream  This makes evolutionary sense…  Ironically there is no blood-ovary barrier, which can actually be a problem since you don't get to "replace" damaged eggs o Placenta (NOT a very effective barrier): usually the blood supplies never mix but they are still close together to allow diffusion of oxygen, nutrients, hormones, etc. HOWEVER, it is found that many substances will cross the placenta  from the mother's blood supply into the fetus' blood supply -- so the placenta is really not much of a barrier at all -- it is useless at restricting the migration of lipophiles Free-plasma and protein-bound xenobiotics  Explain the "binding equilibrium" which XB's exist in. o The big idea is that xenobiotics are carried dissolved in blood in 2 phases:  free plasma phase -- molecules dissolved as solute in water  hydrophiles readily dissolve in water, so they are carried mainly in free plasma phase  protein bound phase -- molecules bound to large plasma proteins (also RBC's possible)  lipophiles tend to bind to plasma proteins, so they are carried mainly in protein bound phase  What kinds of things can XB's bind to, when they are bound? o Plasma proteins Moderately lipophilic: albumin (most common)   Albumin's job is partly to maintain osmotic pressure but also a handy store of protein so if we have a starvation episode, the body can break down some of the albumin  Highly lipophilic: various types of lipoproteins (i.e. chylomicron)  These guys are the heavy hitters who have to carry big time LP stuff like cholesterol  Special carrier proteins (e.g. transferrin for iron and some other metal ions) o Also, we found recently that erythrocytes will allow some partitioning of XB's…meaning that they can be bound within the e-cytes itself…mostly to proteins found in the e-cytes Normal blood capillaries  Describe the features of blood capillaries, and why we care about them in a discussion of distribution. o Capillaries are essentially microscopic tubes made of flat cells arranged into cylinder  The flat cells are capillary endothelium ("inner") cells o In this capillary there are gaps between the cells, which means that the capillaries are FENESTRATED  The gaps are very small (50-100 A) and so glucose, amino acids, hormones, and so on can move through -- but certain things CANNOT, which is important and will be discussed later  The "passing-through" is called PARACELLULAR PERMEATION o The reason we can are about stuff like this in a discussion about distribution is that the road from bloodstream to tissue for a xenobiotic goes through the capillaries -- there are no other ways for the XB's to get out of the system!  Compare and contrast the ease with which hydrophiles vs. lipophiles can do this. o Hydrophiles can pass thru capillary wall into tissue ECF, because they are small enough generally  must be smaller than 100 A o Lipophiles cannot easily permeate capillary wall by paracellular permeation  This is because they are mostly bound to plasma proteins (remember?), and the combined bulk of it is too much  However, they can permeate capillary wall by passive diffusion when they are in the free plasma phase Competition-displacement of two lipophilic drugs  Describe this diagram. o This diagram demonstrated many principles, including:  How lipophilic drugs bind to plasma proteins like albumin  The effect of such binding on the bioavailability of drugs  The effect of adding a new drug that has more affinity for albumin than the first drug o So as we know, drugs (in this case tolbutamide) can bind to proteins like albumin because they have the same characteristics as fatty acids (which is what albumin normally carries) o We notice that as mentioned, drugs that bind highly to albumin won't go into the tissues because they are too big to do paracellular permeation  However, note that there is an equilibrium between free and bound and as the free drugs go into the ECF, some bound ones will become free and so on and so forth until all the drug DOES enter the ECF  So what we are saying is that the albumin doesn't block the drug, but it can slow down its rate of effect o Also when we add another drug (in this case warfarin) which is MORE LIPOPHILIC and thus binds to the albumin better, it displaces all the molecules of the first drug and forces them to be free in the plasma, and thus we see that much more of them are suddenly bioavailable  This is perhaps an example of an additive or interaction effect  Talk about the clinical uses of the two drugs in question. Also, clarify some of the effects mentioned above using the specific properties of these drugs. o Tolbutamide is for diabetes: it is a hypoglycemic drug which stimulates the secretion of insulin from the pancreas o Warfarin is an anticoagulant drug which is used along with tolbutamide to manage diabetes when it becomes advanced, because when this happens people often get blood clots -- typically in the feet and legs o What happens, then, is that the warfarin displaces the tolbutamide from the albumin and thus the tolbutamide's effects increase exponentially  This causes a sugar crisis in the body since there is now so much insulin that all the sugar is removed from the bloodstream and the brain is screwed because it NEEDS that sugar (cannot use fatty acids)  What is the moral of this story? o It is that the big question with a drug/chemical is NOT "is that chemical toxic?" but rather "is the chemical toxic given all the other things that I'm taking right now?" o It's not a question of does one chemical do something but rather "how do they all interact together" -- this is important when we are figuring out what is a safe exposure to a substance Free-plasma and erythrocyte-bound xenobiotics example: lead binding to ALAD protein  Alright, so now we are going to combine this free vs. bound concept with ALAD, which we discussed earlier. Explain the diagram. o Alright, well the idea here is that we are giving another demonstration of how an XB can bind to something in the bloodstream and create an equilibrium between free XB and bound XB o In this case it is the ALAD it is binding to, and so the slide just gave some more detailed context regarding where the ALAD comes from o We see that the ALAD is on the surface of the RBC and it has many thiol (--SH) groups which can lose their protons to become S2-  The thiol groups are from cysteines  This is important because it then can bind to Zn2+, which is an important cofactor for ALAD's role in heme synthesis o Note that within a given ALAD, there is "Site A" and "Site B"  Each site has 4 cysteines, and thus 4 thiol groups o Now the thing is, lead is also a divalent cation (along with zinc) but it has a STRONGER IONIC NATURE than zinc and so it will DISPLACE zinc on the ALAD  Not only this, but it binds to both of the thiols on Site B as zinc, and then ADDITIONALLY binds to a thiol from Site A since it is so affinitive