Pharmacology

Who really decides how much water sits in the gut lumen — that is the first mystery this section unravels. The answer is not that water simply stays because it wants to; it is dragged along by chloride ions. The apical membrane of the intestinal epithelium has two doors: a big one called CFTR, and a small one called ClC-2. The cell actively pumps chloride into the lumen; as negative charge builds up it draws sodium in after it; sodium and chloride together raise the osmotic pressure of the lumen, and water seeps in along the gradient, so luminal fluid increases, stool softens, and motility speeds up. Run it the other way — turn the chloride valve down — and water has no reason to enter the lumen, so secretory diarrhea stops. Constipation needs the faucet opened; secretory diarrhea needs the faucet turned down. These are the two directions of the same causal chain.

The first drug, lubiprostone, is a derivative of prostaglandin E1; it wakes up the normally quiet small faucet, ClC-2. Once the channel opens, chloride floods out, water follows into the lumen, stool softens, and the gut starts moving — so it is used for constipation-predominant IBS, chronic idiopathic constipation, and opioid-induced constipation. It does not touch the big faucet, CFTR, which is the exam's fork in the road; its common side effect is nausea, which is dose-related and eased by taking it with food. If you need a hook, remember the word "lubricate" hidden in its name — lubricating the gut means opening the backup faucet, ClC-2. The second drug, crofelemer, is extracted from a South American plant called dragon's blood; it is one of the rare oral drugs that is almost not absorbed systemically. Its entire working field is the gut lumen, and once it has done its job it leaves with the stool, so systemic side effects are minimal. It inhibits both the big faucet, CFTR, and another channel, the calcium-activated chloride channel, together, so the intestinal epithelium can no longer pump chloride into the lumen; water is held back in the blood, and secretory diarrhea stops. Its approved indication is narrow — noninfectious, drug-induced diarrhea in HIV patients receiving antiretroviral therapy — because that type of diarrhea is driven by exactly this excess-secretion mechanism, which the drug addresses precisely.

The exam loves to attack from three angles here. The first is flipping the direction — choosing crofelemer for what is actually constipation, or lubiprostone for what is actually diarrhea. The fix is to first ask whether the patient has too little water or too much, then follow that logic through to opening or closing the faucet. The second is swapping the target channels — writing lubiprostone's target as CFTR, or crofelemer's as ClC-2. This is pure paired memorization, but as long as you remember that Lubi works the small backup faucet and Crofe works the main faucet, you won't get it wrong. The third is absorption — crofelemer is almost not absorbed systemically, so systemic side effects are rare, yet many people assume it acts throughout the body like an ordinary oral drug and misjudge its safety. Treat the gut lumen as a sink, chloride as the faucet, and the drug as merely something that opens or closes it, and every test point in this section strings together into one line.

The oncology attending draws a clock face on the whiteboard, splitting anticancer drugs into two great camps. Drugs that pick a phase are called cell-cycle specific, because the cancer cell has to arrive at that particular compartment before the drug can kill it — so they are especially effective against rapidly dividing cells, but their effect has a ceiling: no amount of extra dose will save cells that are not in that compartment. Drugs that do not pick a phase are called nonspecific; they damage DNA directly and act throughout every phase, so they are dose-dependent — alkylating agents, platinum compounds, and anthracyclines all sit on this side. This first classification makes clear, at a glance, how dosing logic should proceed.

Place every drug at its corresponding position on the clock, and the exam's favorite traps fall apart on their own. First, irinotecan acts in the synthesis phase, because it inhibits topoisomerase I and interferes with DNA synthesis — do not confuse it with the spindle drugs of the division phase. Second, ixabepilone acts in the division phase; it shares a route with the taxanes, stabilizing microtubules so the cell cannot split apart, but many people mistakenly place it in the synthesis phase. Third, bleomycin is one of the few antitumor antibiotics clearly classified as cell-cycle specific, arresting at G2 — completely different from the anthracyclines, which are also antibiotics but are nonspecific; the two are frequently confused. Cyclophosphamide is the textbook prodrug: it has no activity of its own and must reach the liver to be oxidized by cytochrome P450 before it becomes active. The resulting active metabolite cross-links DNA to kill cancer cells, but the reaction also produces a toxic byproduct, acrolein, which irritates the bladder mucosa on its way out in the urine — that is hemorrhagic cystitis. Prevention relies on mercaptoethanesulfonate sodium binding the acrolein, plus aggressive hydration to dilute it. So when the exam asks for the signature toxicity, the answer is always hemorrhagic cystitis, never cardiotoxicity — cardiotoxicity is the anthracyclines' signature.

6-mercaptopurine is cleared by the garbage truck called xanthine oxidase, and as it happens, the gout drug allopurinol works by inhibiting exactly this truck. So when the two drugs collide, 6-mercaptopurine has no truck left to haul it away, piling up to three or four times its normal blood level, and severe myelosuppression erupts; when used together, the dose must be cut to one-quarter of the original to stay safe. Azathioprine is metabolized into 6-mercaptopurine, so it must be reduced the same way. This is the licensing exam's favorite drug-interaction question, and at its core it is simply two drugs competing for the same metabolic enzyme. Anthracyclines such as doxorubicin inhibit topoisomerase II, causing double-strand DNA breaks while also generating large amounts of free radicals; cardiac myocytes have relatively weak antioxidant capacity, so they collapse under sustained free-radical attack, making the signature toxicity cumulative-dose dilated cardiomyopathy and heart failure. Prevention can rely on dexrazoxane, an iron chelator that strips iron out of the myocardium and reduces free-radical generation. Combining trastuzumab with an anthracycline makes cardiotoxicity additive — another frequently tested trap.

The naming of targeted drugs actually hides the answer. Drugs ending in "-nib" are small-molecule tyrosine kinase inhibitors that strike the intracellular kinase segment of the receptor — for instance, lapatinib strikes the intracellular kinase of HER-2 to treat breast cancer. Drugs ending in "-mab" are monoclonal antibodies that strike the extracellular segment of the receptor or an extracellular ligand — for instance, bevacizumab binds vascular endothelial growth factor to starve the tumor's blood vessels, and trastuzumab strikes the extracellular domain of HER-2, hitting a different segment from lapatinib but used in the same patient population. One oddball is catumaxomab, a bispecific antibody that holds EpCAM on the tumor with one hand and CD3 on the immune cell with the other, drawing T cells alongside the tumor to kill it, used for malignant ascites. The signature toxicities of the other single agents each have their own story you can reason through: bleomycin is pulmonary fibrosis, cisplatin is nephrotoxicity, ototoxicity, and peripheral neuropathy, vincristine is peripheral neuropathy while vinblastine is the one causing myelosuppression, and methotrexate's myelosuppression and mucositis can be rescued with leucovorin. Tie every drug to its mechanism and toxicity along the causal chain, and when the question arrives you will not need to memorize it — you will simply reason it through.

A drug-drug interaction always splits first into two levels. A pharmacokinetic interaction changes the blood concentration, affecting any one of the four stops — absorption, distribution, metabolism, excretion; a pharmacodynamic interaction changes the drug effect, for example two antihypertensives adding together, or a benzodiazepine adding to alcohol's central depression. In one line: kinetics changes the amount of drug, dynamics changes the drug's effect. The liver's cytochrome P450 is the body's largest metabolic kitchen; if another drug inhibits it, the substrate drug gets stuck in gridlock, its blood concentration piles up, and it heads toward toxicity; if another drug induces it, the substrate is cleared too fast, its concentration drains away, and its efficacy fails. Classic inhibitors include cimetidine, ketoconazole, erythromycin, and grapefruit juice; classic inducers include rifampin, phenytoin, carbamazepine, barbiturates, and chronic alcohol use — this is exactly why they cause oral contraceptives and warfarin to fail. Picture clearance as the kitchen's drainage rate, and volume of distribution as the house the drug lives in; link the two to the half-life, which equals 0.693 times the volume of distribution divided by clearance — the slower the drainage or the bigger the house, the longer the half-life.

The ironclad rule of the renal tubule is just as simple: an ionized drug cannot cross the cell membrane and is swept away in the urine, while a nonionized drug is instead reabsorbed back into the blood. So for acidic-drug poisoning such as aspirin or phenobarbital, give sodium bicarbonate to alkalinize the urine; the weak acid ionizes in the alkaline environment, is not reabsorbed, and clears faster. Conversely, to speed the excretion of a basic drug you would acidify the urine, though this is rarely done clinically. Remember "treat acid poisoning with base," and you will never flip the direction into treating acid poisoning with acid or base poisoning with base. Chelator pairing for heavy-metal poisoning is the licensing exam's easy-marks question that is nonetheless the easiest to get wrong — the key is not to misuse penicillamine. Arsenic poisoning calls first for DMSA, DMPS, or dimercaprol; lead poisoning calls for EDTA or DMSA; mercury poisoning calls for DMSA, DMPS, or dimercaprol; copper poisoning — that is, Wilson disease — is the only one that calls for penicillamine or trientine; iron poisoning calls for deferoxamine. The exam loves to slip you the wrong pairing of penicillamine for arsenic poisoning; just remember the line "arsenic gets no penicillamine, only copper gets penicillamine."

The story of disulfiram, a drug that supports abstinence, is an elegant metabolic chain. Alcohol is oxidized in the liver by alcohol dehydrogenase into acetaldehyde, and acetaldehyde is then oxidized by acetaldehyde dehydrogenase into acetic acid and cleared — a two-step assembly line. What disulfiram inhibits is the second step's acetaldehyde dehydrogenase, so acetaldehyde gets stuck and accumulates, causing nausea, vomiting, flushing, palpitations, headache, and hypotension; the patient is frightened enough that the next time they smell alcohol, they are afraid — which is exactly the principle behind its use in supporting abstinence. Other drugs that trigger the same reaction include metronidazole, cephalosporins with a specific side chain such as cefamandole and cefotetan, chlorpropamide, and griseofulvin — this group is a high-frequency, easy-marks question: see a patient on these drugs who drinks and flushes, follow the causal chain of inhibited acetaldehyde dehydrogenase, and the answer pops out. Cimetidine, although it is an H2 blocker used to treat peptic ulcers, carries an antiandrogen side effect; long-term use causes impotence and gynecomastia in men, and it is itself a cytochrome P450 inhibitor with a long list of interactions — so clinically, famotidine is now preferred, since it has the strongest acid-suppressing potency and none of cimetidine's antiandrogen problems, making it the first-choice H2 blocker.

The last point that is easy to overlook but always tested is pre-treatment genetic screening. Abacavir is a first-line drug in HIV treatment, but patients carrying the HLA-B*5701 allele develop a fatal hypersensitivity reaction, so HLA-B*5701 must always be tested before treatment, and a positive result contraindicates the drug. Easily confused with it is the carbamazepine group, which requires testing for HLA-B*1502 — especially in Asian populations — to avoid Stevens-Johnson syndrome. Hold on to three main threads and this whole section falls into place: kinetics changes the amount of drug, dynamics changes the drug's effect; CYP inhibition causes gridlock and toxicity, CYP induction clears the shelves and causes failure; and among the antidote pairings, penicillamine goes only to copper — never hand it to arsenic.

The emergency department admits a patient with phenytoin toxicity: adding just one extra pill sent the blood concentration surging to three times normal, with nystagmus, ataxia, and slurred speech, and only when the resident checked the textbook did it come back to him that this drug is eliminated by zero-order kinetics, where a small increase in dose causes a surge. Next door, anesthesiology is using desflurane for an outpatient surgery, for a simple reason: its blood:gas partition coefficient is small, so induction is fast and recovery is fast too. Both departments are running on the same set of pharmacokinetic equations, and this section is about taking that set of equations apart. The core purpose of biotransformation is to convert a lipophilic drug into a water-soluble metabolite so it can be excreted by the kidney or bile, but it is not necessarily detoxification — some metabolites are in fact more toxic, and some prodrugs cannot work at all without being switched on by the liver. Phase I is oxidation, reduction, and hydrolysis, with cytochrome P450 as the lead actor; the product may still be active, and a prodrug may even be activated at this very step. Phase II is conjugation — attaching glucuronic acid, sulfate, an acetyl group, or glutathione to ship the drug out of the body, usually inactivating it and greatly increasing its water solubility. Remember it as: Phase I opens a door and installs a handle, Phase II attaches a label and ships it out. The absorption of weak acids and weak bases is worked out from the Henderson-Hasselbalch equation. The key point is that a drug can cross the cell membrane and be absorbed only when it is nonionized; the ionized form is trapped where it is. One example you must know by heart: a weak acid with a dissociation constant of six, placed in a stomach at pH four, is two log units away, so ionized to nonionized is one to a hundred — the nonionized form makes up the overwhelming majority, and it is readily absorbed in the stomach, which also matches the mnemonic "acid in acid, base in base, nonionized." Its mirror image is treating acid poisoning with base: put an acidic drug into alkaline urine on purpose, letting it ionize and get flushed away, speeding its excretion. The key to inhaled anesthetics is the blood:gas partition coefficient. A large coefficient means the gas loves to dissolve in blood, so it takes a long time for the brain to receive a high enough partial pressure, making induction slow and recovery slow too — halothane is like this. A small coefficient means the gas does not dissolve much in blood, so blood partial pressure rises quickly, giving fast induction and fast recovery — both desflurane and nitrous oxide are like this. Picture the gas as a salesman and the blood as a lobby: the clingier the lobby, the harder it is for the salesman to fight through to his destination, the brain — keep that direction in the metaphor and you will not get it wrong.

The four types of antagonism can be told apart just by looking at the curve. Competitive antagonism binds the same site, reversibly, and shifts the agonist's curve to the right, but the maximal efficacy stays the same, so raising the agonist's concentration can push it back. Noncompetitive antagonism binds irreversibly, or at a different site, pressing down the ceiling of the curve — the maximal efficacy falls, and no amount of added agonist can rescue it. Chemical antagonism does not bind the receptor at all; the two drugs simply meet and inactivate each other directly, as when protamine neutralizes heparin. Physiologic antagonism acts through different receptors but produces opposing physiologic effects, as with epinephrine against histamine — in anaphylactic shock, epinephrine constricts blood vessels while an antihistamine blocks the receptor instead. Of these four types, the most frequently tested distinction is the curve behavior of competitive versus noncompetitive antagonism; remember competitive as a reversible rightward shift and noncompetitive as a lowered ceiling, and you are set. For bioavailability, an intravenous injection is always one hundred percent, because it enters the blood directly with no first-pass effect, whereas an oral dose is discounted by both absorption rate and hepatic first-pass metabolism, so an oral dose is always less than one hundred percent.

Elimination kinetics is the most important test point in this section. Most drugs follow first-order kinetics: a fixed proportion is metabolized away, and the half-life is fixed, independent of concentration. A small handful of drugs follow zero-order kinetics: a fixed amount is metabolized away, and the half-life lengthens as concentration rises, because the metabolizing enzyme is already saturated and there is a ceiling on how much it can carry away. The representative drugs spell out PEA: phenytoin, ethanol, high-dose aspirin. The reason phenytoin absolutely must have its blood level monitored is that, within the therapeutic range, it is already zero-order — a small increase in dose can send the blood concentration surging from therapeutic straight into toxic, bringing nystagmus, ataxia, even coma. The fact that you cannot drive after drinking follows the same logic: blood alcohol falls by a fixed amount per hour, regardless of how much you drank. Picture the volume of distribution as the house the drug lives in — a large volume means the drug has run off to hide in the tissues, leaving little in the blood; picture clearance as the kitchen's drainage rate — how large a volume of plasma is cleared of drug each minute. The half-life equals 0.693 times the volume of distribution divided by clearance, so the larger the volume of distribution or the smaller the clearance, the longer the half-life. Finally, personalizing the dose of warfarin depends mainly on two genes: CYP2C9 governs its oxidative metabolism and clearance, and VKORC1 is the target enzyme it acts on. A CYP2C9 variant slows metabolism, raising bleeding risk and requiring dose reduction; a VKORC1 variant alters sensitivity — you need to look at both together to get it right. The exam loves to slip in CYP3A4 or CYP2D6 to trick you; neither has anything to do with warfarin, so do not pick them by mistake.