[ Please welcome our guest blogger, who identifies as robin, just your average everyday neuropharmacologist. -DM ]
One of the most important yet overlooked tasks of the average pharmacologist is dissolving drugs into solution. Those of you who work with things that don't have to cross the blood-brain barrier probably have a generally easier time dissolving shit than those of us who prefer to study CNS-active compounds. For those of us who play with compounds that are hydrophobic enough to cross the blood-brain barrier, I can testify that those range from fairly easy to major suck to put into an aqueous solution.
The principal reason this is important is pharmacokinetics. I almost said simple pharmacokinetics, but those two words really only belong together when the intent is sarcastic. In the in vivo model, one has to contend with the processes of absorption and distribution before the drug can go to work. Absorption, being the first step, is crucial.
In order for a drug to do its thing, it first has to be absorbed into the bloodstream. A typical experimental route of administration is through the peritoneal cavity, or i.p. This is not injecting directly into the bloodstream, but there is quite a bit of blood circulating in the general vicinity. The drug has to pass through all of the membranes standing between it and the blood in order to begin to circulate to its site of action. And once we've got this batch of drug molecules running around in the bloodstream, presuming they got there in the first place? They're hanging out in the fat cells, or not. They're slowly (or quickly) making their way to the site of action. And that's not even considering the metabolism and excretion parts of pharmacokinetics! You see there are many variables to in vivo pharmacology, even under the most optimal conditions.
So what happens when things go wrong? Say the drug is not dissolved. It's decided it hates you (ok, probably too much anthropomorphizing, but at this point I might as well go for it) and when you add anything the least bit aqueous, it crashes out of solution into crystallized form. Convenient way to isolate a drug, perhaps- shitty way to prepare one for use. If this happens and you try to gather some of this shit up and inject anyway, well, you're doin it wrong. The drug needs to be in some kind of homogeneous solution.
Why do we care? Pharmacologists like to see a drug concentration-response effect when looking for drug effects. Drug concentration is a measure of how much drug is dissolved in solution. Using an incompletely dissolved drug will not reflect the drug concentration, but instead how many crystals comprised of how many milligrams (or fractions thereof) that you randomly collected for that particular syringe draw- if you collected any at all. Dissociation of molecules from the crystallized form into the body compartment is a major rate-limiting step in absorption. This is a crucial step in the oral dosage format, but injection of solutions is intended to bypass some of the common problems of oral dosing. In this case, the intended, effective drug concentration might never be reached.
But there are more reasons than just the theoretical science here. The crystal might not fit through an appropriately sized needle, for one. And pushing a crystallized drug can cause undue pain to the subject, which is to be avoided at all costs. These are not secondary concerns; they are non-negotiable from an ethical standpoint.
So what do you do? There are ways around it- rain dances that we do. Some dissolve hydrophobic drugs straight up in some kind of oil and inject as an oil depot. This is a slower pharmacokinetic situation, because the drug equilibrating between the oil and the body is a huge rate-limiting step. Hydrophobic interactions between drug molecules can slow absorption considerably. But it's useful for some types of studies. In general, aqueous solution is preferred. Why? Most simply, we want the drug solution to be miscible with the aqueous body compartment that receives the injection. But we don't like to add pure water into biological systems. Pure water screws with the delicate ionic balance maintained by cells, and causes major trouble. Saline is the default water-based injection vehicle. Creating an aqueous solution sometimes requires one to fuck around for ages (in bench-science time) finding the lowest percent of [choose your oil-based substance] that will keep the drug in solution. And in some cases, you can dilute a hydrophobic drug in a non-aqueous solvent far enough that adding aqueous liquid slowly and carefully will not cause it to crash out of solution. It all depends on the drug and the concentration you're looking for, but it's usually a task.
What about emulsions? Consider propofol, a short-acting intravenous anesthetic and recent headline-maker in the death of Michael Jackson. Propofol is totally fucking hydrophobic. You will not for the life of you get that shit into a water-based solution. In this case, in order to prepare the drug sufficiently for use in people, it is dissolved in oil first. The drug dissolved in oil is mixed with aqueous solution and surfactants, and processed to a uniform oil droplet size and dispersion in the aqueous solution. The surfactants keep the oil droplets suspended in the water-based solution, whereas oil droplets would typically merge and separate from solution without surfactants. The small size of the oil droplets increases drug surface area, improving drug-blood equilibration time. (note that i.v. means the drug goes straight into the bloodstream.) This is the next-best thing because the amount of drug is evenly distributed through the solution. If it was not properly distributed, physicians would not know whether they were giving a proper dose to maintain sedation and anesthesia. With any drug concentration unknown or outside a certain error margin, major problems arise. With propofol, patients might wake up during surgery without sufficient drug administration- or conversely, they might overdose with too much. Both are very much undesirable, and this potential source of error is considerably reduced due to proper procedure in dissolving drug.
While we experimental pharmacologists may work in very different settings and conditions than, say, your average anesthesiologist- the principles of drug dissolution remain the same. In order for the drug to work for us, we have to work with the drug to get it into solution. Taking the time and trouble to do it right matters.