VTNE

Pharmacy and Pharmacology is worth 20 of the 150 scored items (13%). It covers how drugs are absorbed, distributed, metabolized, and excreted; how they act on receptors; the major veterinary drug classes and their species-specific hazards; dose, percent-solution, fluid, and constant rate infusion calculations; and the legal rules for labeling, dispensing, storing, logging, and disposing of drugs. Expect calculation items with four close numeric answers, "which drug reverses…" and "which species is at risk…" recall items, and scenario items about DEA forms, controlled-drug logs, and extra-label use in food animals.

Pharmacology is the study of drugs: their origins, chemical properties, biological effects, and therapeutic uses. Pharmacokinetics describes what the body does to a drug, summarized as ADME: absorption, distribution, metabolism, and excretion. Pharmacodynamics describes what the drug does to the body.

➜

➜

➜

Site of administration into the

Bloodstream to body tissues

Biotransformation, mainly in the

Mainly kidneys; also bile, lungs,

bloodstream

liver

milk

Absorption and Distribution

Absorption depends on the route of administration, the drug formulation, blood flow to the absorption site, and the drug's lipid solubility. Oral drugs must cross the gastrointestinal tract and pass through the liver in the portal blood, where first- pass metabolism may inactivate part of the dose before it reaches systemic circulation. Injectable routes bypass the GI tract and give faster, more predictable absorption.

In the blood, many drugs bind plasma proteins, primarily albumin; only the unbound (free) fraction is pharmacologically active. Highly protein-bound drugs have a smaller volume of distribution, and hypoalbuminemic patients may show exaggerated effects. The blood-brain barrier admits mainly lipid-soluble drugs, and the placental barrier restricts fetal exposure but is not absolute.

Routesofadministration,Fastesttoslowestonset

Route	Absorption	NursingNotes
Intravenous(IV)	Immediate;100%	Mostpredictable;irritatingdrugs(thiobarbiturates phenylbutazonein ,
	bioavailable	horses)causetissuesloughingifgivenperivascularly
Intraosseous(IO)	NearlyasfastasIV	Marrowcavityaccess(proximalhumerus femur tibia)inneonates small , , ,
		exotics orcollapsedpatientswhenaveincannotbefound ,
Inhalation	Rapidacrossalveoli	Inhalant anesthetics; nebulized or metered -dose bronchodilators via s1pacer andmask

Routesofadministration,Fastesttoslowestonset(Continued)

Route	Absorption	NursingNotes
Intramuscular(IM)	Fast(goodbloodsupply)	Epaxial quadriceps ortricepsinsmallanimals;neckor , ,
		semimembranosus/semitendinosusinhorses;neckinfoodanimalstoprotect
		valuablecuts
Subcutaneous(SC)	Slower;reducedifpatient	Looseskinoverthedorsalneckandshoulders;usedforfluids vaccines , ,
	isdehydratedorinshock	manyantibiotics
Oral(PO)	Slowestandmostvariable;	Affectedbyfood GImotility andchelatingagents;notforvomitingor , ,
	first-pass effect	obtundedpatients
Topical/transdermal	Slow;localorsystemic	Weargloves;transdermalgels(methimazole)andpatchesdeliversystemic
		drugthroughtheskin

Diazepam is absorbed erratically and painfully after IM injection; midazolam, which is water soluble, is the benzodiazepine chosen for IM use. Several emergency drugs (naloxone, atropine, vasopressin, epinephrine, lidocaine) can be given intratracheally when no vascular access exists.

Metabolism and Excretion

Metabolism (biotransformation) occurs mainly in the liver. Phase I reactions (oxidation, reduction, hydrolysis) are carried out largely by cytochrome P450 enzymes; Phase II reactions conjugate the drug to make it water soluble. Prodrugs require metabolic activation (prednisone to prednisolone, enalapril to enalaprilat). Enzyme induction (phenobarbital) speeds metabolism and reduces drug effect; enzyme inhibition slows metabolism and can cause toxicity. Cats have limited glucuronide conjugation, which explains their sensitivity to acetaminophen, aspirin, and permethrin.

Excretion occurs mainly through the kidneys (water-soluble compounds) and through bile into feces. In enterohepatic recirculation, drug excreted in bile is reabsorbed from the intestine, prolonging its action. Volatile anesthetics are excreted through the lungs; drugs also pass into milk and saliva, which is why food-animal drugs carry milk withdrawal times.

Pharmacodynamics: Receptors and Response

Drugs act on receptors, enzymes, ion channels, or cell membranes. Affinity is how tightly a drug binds its receptor; efficacy is the maximum response it can produce; potency is the amount of drug needed for a given effect.

Receptor	EffectWhenStimulated	ExampleDrugs
Alpha-1	Vasoconstriction	Phenylephrine(agonist)
Alpha-2	Sedation analgesia reducedsympatheticoutflow , ,	Xylazine dexmedetomidine;reversedby ,
		atipamezole
Beta-1	Increasedheartrateandcontractility	Dobutamine(agonist);atenolol(blocker)
Beta-2	Bronchodilation	Terbutaline albuterol clenbuterol , ,
Muscarinic	Bradycardia salivation GImotility , ,	Atropine glycopyrrolate(blockers) ,
Muopioid	Analgesia sedation respiratorydepression , ,	Morphine hydromorphone fentanyl; , ,
		naloxonereverses
GABA-A	Neuronalinhibition sedation anticonvulsanteffect , ,	Benzodiazepines barbiturates propofol , , ,
		alfaxalone
NK1	Vomiting(substanceP)	Maropitant(antagonist)

Dose-Response, Therapeutic Index, and Half-Life

The therapeutic index (TI) is the ratio of the toxic dose to the effective dose (classically LD50 ÷ ED50). Drugs with a narrow TI (digoxin, aminoglycosides, theophylline, potassium bromide) require careful dosing and serum-level monitoring. Half-life (t½) is the time for plasma concentration to fall by 50%. With repeated dosing, a drug reaches steady state after about five half-lives, and about five half-lives are also needed to eliminate most of the drug after the last dose; a loading dose reaches the therapeutic range sooner.