How a drug works at a Molecular level?

Question

Say a Drug is injected in the Body with a certain quantity (usually 2 - 5 cc) contains x no of molecules. The body contains y no of cell and obviously y >> x. That means each cell will not get the molecules. Moreover there are transit loss due to adheson with arteries and vessels, and musculor absortption. Still how does it work to cure illness ?

Please suggest some good internet reference as well

Answer 1

How the drug work depends on the drug.  Some work by inhibiting a receptor.  They bind to it and prevent the natural agonist from working.  An example is beta blockers.  They block beta adrenergic receptors and relax cardiac muscle, thereby lowering blood pressure and decreasing the hearts workload.  Other drugs stimulate receptors such as opiates like morphine.  Morphine stimulates the mu-opiate receptor and causes sensory neurons to hyperpolarize and make them less excitable to stimulus.  Some other drugs inhibit or excite enzymes, act directly on mRNA or tRNA, or effect ion channels.

And drugs don't adhere to arteries and other blood vessels (except clotting agents).  A major consideration pertaining to what you're alluding to is known as pharmacokinetics; ie, what the body does to the drug.  Some drugs are poorly absorbed by the digestive system (morphine), or are destroyed by it entirely (insulin).  Some drugs are actually inactive when they are ingested and are converted to an active form by metabolism (L-dopa).  The main thing is that doses administer enough drug to exert the desired effect without causing toxicity.  

I hope I answered your question, and if you want any specific info I'll be glad to help.

Answer 2

This is a really complex question with no simple answer. But maybe I can break it down into a few basic parts. Basically, the assumption that y>>x is wrong. Actually, with most drugs that are distributed systemically, x>>y.

Let's say you sprain your ankle and it starts to hurt/swell. So you go to your medicine cabinet and take 600mg of ibuprofen. The molecular weight of ibuprofen is 206.3g/mol, which means that you're taking 3mmol of ibuprofen, or 1.8e21 molecules.

The bioavailability of ibuprofen is 49-73%. This is how much ibuprofen actually gets into the blood after digestion and absorption. So were down to about 1e21 ibuprofen molecules actually getting into the body. But, the number of cells in the human body is on the order of 1e14. This means that you're taking about 10 million ibuprofen molecules for every 1 cell in the body. That's a lot!

Now, you're right, some of these molecules are immediately excreted, some are metabolized, and some get stuck (for a time) on proteins in the blood. However, a large proportion of those molecules will diffuse or be transported to different parts of the body. Certain parts won't take up some drug. For example, ibuprofen won't penetrate into bone or really get into adipose tissue. So the rest of the parts of the body will get a correspondingly higher dose.

In the end, the drug will travel where it can, and build up in all sorts of tissues before becoming metabolized or excreted. We're hoping that your ibuprofen gets down into that swollen tissue in your ankle and exert pharmacological effect on the inflammation and pain there, where certain proteins and signals are turned on. And we know from trial and error that the dose we give will be enough, so the pain and swelling is lessened.

We haven't talked about what happened to the ibuprofen that gets placed elsewhere in the body - but it's there. You just don't notice it because you're not in pain there. It gets metabolized and excreted over time, just like everywhere else. I should add that when drug companies talk about "side effects" what you're really seeing is the drug working on parts of the body you don't want it to. A great example is Benadryl (diphenhydramine), which works as an antihistamine. This is great for your nose, but when the drug gets into the brain, the same antihistamine activity causes sedation/sleepiness.

Answer 3

I can't think of any illness in which every cell in the body would need to be treated, so the math you present is misleading.  If you are treating an infection with an antibiotic, the drug simply has to reach the site of infection at a concentration high enough to kill the bacteria.   If you were injecting epinephrine to treate an asthma attack, you'd only need to get enough molecules to the lung to activate sufficient numbers of adrenergic receptors.   I would think in terms of the concentrations of the drug that can be reached in the target tissue, not the number of molecules of drug vs. the number of cells in the body.

Answer 4

Medicine usually doesn't "Cure Illness" as you stated.  It really masks the symptoms of the malady until the body conquers what is wrong.  This can be done in a few ways.

1) The medication only reacts with certain receptors found in a certain area of the body (say the nervous system).  That way pain can be endured during healing if the pain is blocked by the medicine on the receptor.

2) Often with allergies and roaming illnesses of the blood, all that is necessary for a person to feel better is a reduction of the antigen in the bloodstream.  This can be accomplished if the medicine attacks, or binds to (inactivates) the antigen (like histamine) before it reaches the receptors in the body.  You think the medicine went to the sinuses because they feel better, but it stopped the histamine before it reached the affected areas instead.

3) Some meds, depending on how they are absorbed by the body, can only go to the affected area.   Thus quicker relief.

Enjoy!