Pharmacology Flashcards
What is the primary site for most oral drug absorptions?
Small intestine
What is the first pass effect?
A phenomenon of drug metabolism at a specific location in the body which leads to a reduction in the concentration of the active drug, specifically when given PO, before it reaches the site of action or systemic circulation
What method of drug administration skips first pass metabolism?
TM and rectal allow for absorption through GIT segments that are not drained by the portal circulation
Why is respiratory drug administration highly effective?
The alveoli are thin with excellent surface area and lined heavily with capillaries, allowing a drug to rapidly and directly enter systemic circulation
What 4 factors is drug distribution dependent on?
- Physicochemical properties of the drug
- Concentration gradient established between blood and site of infection
- Ratio of blood flow to site of action
- Affinity of the drug for tissue constituents
Where are lipid soluble drugs distributed to?
All body water, therefore found in both the ICF and ECF
What are water soluble drugs distributed to?
- Only in the ECF
- Important to remember that the volume of ECF is altered in disease
- In hypovolemia, the ECF shrinks as the depleted water volume shifts to the ICF = increased concentration of drug confined in ECF = potential toxicity
- In edema, ECF volume increases = lowers concentration of drug = subtherapeutic level
What effect do liver shunts have on drug distribution?
Shunts prevent the distribution of drugs to the liver for essential metabolism which can result in toxicity if the drug if the drug is active and remains in circulation, or in failure of therapy if drug relies on biotransformation in liver
What is the MDR1 gene?
- P-glycoprotein 1 or ATP binding cassette subfamily B member 1 (ABCB1)
- A protein (drug transporter) highly distributed within GIT, kidney, liver, and BBB that can remove foreign substances, including drugs, out of cells
- It can transport substances from brain tissue back into capillary lumen, limiting brain exposure to drugs
- Certain breeds lack full expression of the MDR gene and are susceptible to toxic brain exposure/increased sensitivity to drugs
- Herding dogs, aus shepherds, collies
- Avermectins, chemo, loperamide
How does hypoalbuminemia affect drug efficacy?
- Drugs can exist bound to plasma proteins = inactive to the body, or unbound to plasma proteins = active and available to cross cell membranes
- Drugs must be in the unbound state for metabolism and elimination
- The fraction of unbound/active drug may shift and produce a greater effect
What is the cytopchrome P450 (CYP) dependent system?
- The primary microsomal enzyme system responsible for the oxidative metabolism of endogenous compounds
- It catalyzes a diverse range of oxidative reactions
- It is essential for the metabolism of many medications
- CYPs are expressed in liver (primary), intestine, brain, kidneys
- This system can convert drugs to inactive metabolism, active prodrugs, or create toxic metabolites
- Any time 2 drugs rely on CYP metabolism there is the potential for drug interaction
What is glucuronidation?
- A metabolic pathway that involves the conjugation of glucose with other compounds to form glucuronides, which are then excreted in urine and bile
- An elimination and detoxification mechanism for a host of lipophilic compounds
Why do cats have a decreased ability to effectively perform phase II metabolism?
- Cats can’t synthesize glucuronic acid to the extent needed to metabolize some drugs, leading to toxicity
What is an enzyme inducer?
- A drug that can activate or enhance P450, resulting in increased drug mnetabolism
- Pheno, St. John’s Wort
How are most drugs eliminated from the body?
Via the urine through the kidney by glomerular filtration
What form must drugs be in to be eliminated?
Drugs must be water soluble, smaller in molecular weight, polar, ionized, and unbound to plasma proteins to be eliminated
Why are kidneys susceptible to drug induced toxicity?
Due to their high blood flow, tendency to concentrate drugs in glomerular infiltrates and passively reabsorb them
What is enterohepatic recycling?
Some drugs are eliminated in the bile and can be reabsorbed later in the GIT and re-enter the circulatory system
What is the purpose of a CRI?
To administer new drug to the body at the same time the drug is being metabolized and eliminated, thus ‘ins and outs’ is balanced
Why should a loading dose be given when starting a CRI?
In the absence of a loading dose, a CRI will reach steady state plasma concentrations in 3-5 drug half lives
What is pharmacokinetics?
- The study of absorption, distribution, metabolism, and excretion/eliminating of drugs and their metabolites
- What the body does to the drug
What is pharmacodynamics?
- The study of the responsiveness of receptors to a drug and the mechanisms by which these effects occur and impact the body
- What the drug does to the body
What is a ligand?
- Any chemical that binds to receptors: drugs, hormones, neurotransmitters
- When a ligand and receptor bind, a cellular response occurs which serves a biological purpose
What is chelation?
Chelating agents work independently of receptors. They bind directly to heavy metals in the body and promote their excretion
Why can drug distribution be altered in pediatrics and geriatrics?
- Due to increased total body water, decreased fat stores, and hypoproteinemia
- Pediatrics may have fewer P450 enzymes and decreased ability to perform glucuronidation, making drug metabolism immature
- The pediatric renal system is not fully developed until ~ 8 weeks, which may decreases drug clearance
- Pediatrics may have increased BBB permeability, giving some drugs more profound CNS effects
- Geriatrics are at increased risk for CKD, which reduces GFR and decreases drug clearance
How does apomorphine induce emesis?
It stimulates the CRTZ to induce emesis as a nonselective dopamine agonist
Why is apomorphine highly effective in dogs?
- Histamine and dopamine are significant neurotransmitters in the canine CRTZ, making dopaninergic and histaminergic receptor antagonists important antiemetic classes
Why is apomorphine less effective in cats?
- Alpha 2 adrenergic and 5HT3 serotonergic receptors are the significant neurotransmitters in the feline CRTZ
- Cats have fewer dopamine receptors
Why is xylazine the emetic drug of choice for cats?
It is an alpha 2 adrenergic agonist and stimulates the feline CRTZ
How do phenothiazine derivatives exert their effects as antiemetics?
They block dopamine receptors in the CRTZ
How does Ondansetron reduce nausea/emesis?
It is a serotonin antagonist
How does maropitant work?
It is a neurokinin-1 (NK) antagonist that blocks substance P (a neurotransnitter) at the VC
Why should maropitant only be given for 5 days?
It utilizes two P450 enzymnes, one of which has low capacity, leading to accumulation
What effect can maropitant have in dogs under 8 weeks old?
Bone marrow suppression
How do proton pump inhibitors work?
They inhibit Na+/K+-ATPase pump activity, thereby controlling proton deposition into the gastric lumen and hydrochloric acid production
How does omeprazole work?
Binds to the secretory surface of parietal cells, inhibiting transport of H+ ions into the stomach
How do H2 histageneric antagonists exert their effects?
They are specific antihistamines that reduce the action of histamine at histamine receptors on gastric parietal cells, thereby reducing stomach acid production
All H2 antagonists can cause ____ if given too rapidly IV
Bradycardia
Cimetidine is a known P450 enzyme ____?
- Inhibitor
- Decreased metabolism, prolonged 1/2 life, and increased serum level of many drugs
How can Ranitidine exert a weak prokinetic effect?
Due to inhibition of acetylcholinase, thereby stimulating parasympathetic innervations of gastric and intestinal motility
How does sucralfate work?
- It prevents stomach acid from reaching the site of injury and reduces breakdown of mucus
- It promotes bicarbonate production and may increase production of prostaglandins
What is misoprostol?
- A synthetic prostaglandin analogue
- Improves gastric blood flow, decreases gastric acid production, increasing mucus production, and bicarbonate secretion and promoting cell turnover, leading to accelerated healing of gastric ulcers
- Used to prevent NSAID induced GI injury and ulceration
Why is a furosemide CRI superior to intermittent boluses?
Creates more diuresis and Na & Ca excretion with less K loss
Where does Spironolactone exert its effect?
- It competitively binds to its receptor site in the distal tubule and collecting duct
- Permits Na, Ca, & H2O excretion but limits K excretion
Where do thiazide diuretics exert their effects?
They inhibit the Na Cl ion transporter within the distal tubule of kidney, enhancing excretion of H2O, Na, Cl, K, and Mg
How does pimobendan exert its effect?
- Phosphodiesterase III inhibitor that increase the concentration of intracellular Ca, ultimately enhancing the cardiac contractility and cardiac output
- Does not increase myocardial oxygen consumption
- An inodilator: combo of positive inotrope and vasodilation
How do ACE inhibitors exert their effect?
By blocking the conversion of Angiotensin I to II, which lowers arteriolar resistance, increases venous capacity, increases natriuresis, decreases BP, and decreases ventricular remodeling and ventricular hypertrophy
How do the calcium channel antagonists exert their effects?
- Classified as dihydropiridines or non dihydropiridines; bind to different sites to the Ca channel
- Non-dih: more myocardial selective, tend to reduce HR (Diltiazem)
- Dih: more vascular selective, dilates vascular smooth muscle, reduces afterload, lowers BP
How do nitrates exert their effects?
- The treatment of fulminant CHF often depends on rapidly reducing vascular resistance, reducing afterload, and improving cardiac function
- The nitrate drugs, sodium nitroprusside and nitroglycerin, are metabolized in the body to form nitric compounds
- Their production and subsequent diffusion into vascular smooth muscle cells mediate relaxation, venodilation and vasodilation, thus reducing afterload
How does nitroprusside exert its effect?
- Immediately produces a decrease in SVR, which can cause a decrease in arterial BP
- Reduces afterload
- Used for acute hypertensive crisis or fulminant CHF
- Can be used in conjunction with inotropes
- Easily titratable