Deciphering Agonists and Antagonists on the USMLE Step 1 Exam

Tyler York
usmle step 1 agonists and antagonists

If you’re studying for the USMLE Step 1, understanding the pharmacodynamics of drugs and how they interact with patients is crucial. In the below video, Sujata Arecanteparamb, M.D., founder of GraceUSMLE and author of Achievable’s USMLE Step 1 course, explains the difference between agonists and antagonists in this short video.

If you’re looking for a comprehensive course to maximize your score on the USMLE Step 1 exam, check out Achievable’s USMLE Step 1 course that was authored by Sujata.

Full Agonists and Antagonists transcript below:

00:00
 in this video i want to talk with you
 00:02
 about a few
 00:03
 aspects regarding pharmacodynamics of a
 00:05
 drug
 00:06
 do you know the difference between
 00:08
 agonist and antagonists
 00:10
 do you know how agonists and antagonists
 00:13
 affect the
 00:14
 efficacy and potency of a drug let's
 00:16
 look at these terms
 00:18
 this concept is important to understand
 00:20
 drug receptor interactions for the usmle
 00:23
 let's begin with agonist agonics or also
 00:26
 called as
 00:27
 full aegonis are drugs which bind to a
 00:30
 receptor
 00:31
 and then produce the maximum response or
 00:34
 they produce the desired response an
 00:37
 example
 00:38
 can be epinephrine when epinephrine
 00:41
 binds to the beta-1 receptor
 00:43
 it produces the desired response which
 00:45
 is increase in heart rate
 00:49
 now let's take a look at the different
 00:50
 kind of aegonis
 00:52
 when a partial agonist binds to the
 00:55
 receptor
 00:56
 it produces less than maximal response
 00:59
 or in other words
 01:00
 less than 100 percent response as that
 01:03
 produced by a full agonist a partial
 01:06
 agonist
 01:08
 has lower efficacy than a full agonist
 01:12
 sometimes when a partial agonist is
 01:14
 given
 01:15
 in the presence of a full agonist it may
 01:17
 behave
 01:18
 like an antagonist just because of the
 01:21
 reason
 01:22
 that both of the drugs will compete or
 01:25
 bind
 01:26
 to the same receptor some partial
 01:28
 agonists can behave sometimes like full
 01:31
 aegon is
 01:32
 or even like antagonists depending upon
 01:35
 which receptor they are using
 01:37
 an example will be pentazosine
 01:40
 pentazosine
 01:41
 is a full agonist at the kappa receptor
 01:45
 but it is a partial agonist at the mu
 01:48
 receptor
 01:49
 now let's look at inverse agonist when
 01:52
 an
 01:52
 inverse agonist binds to the drug
 01:54
 receptor it produces an effect which is
 01:57
 opposite
 01:58
 to the effect produced by a full agonist
 02:01
 for example if the agonist increases the
 02:04
 heart rate
 02:05
 binding of the inverse agonist will
 02:08
 cause
 02:08
 a decrease in the heart rate in the
 02:11
 presence of a full agonist
 02:13
 an inverse agonist behaves like an
 02:15
 antagonist
 02:17
 an example will be the effect of
 02:19
 naloxone
 02:20
 on the mu receptors now let's take a
 02:23
 look at the different types of
 02:25
 antagonists an antagonist by itself
 02:29
 does not have any inherent activity on
 02:32
 the receptor
 02:34
 when an antagonist binds to a receptor
 02:37
 it simply prevents the agonist from
 02:39
 binding to the receptor
 02:41
 an example would be propranolol which is
 02:44
 a beta blocker
 02:46
 when it binds to the beta-1 receptor
 02:49
 it prevents binding of epinephrine to
 02:52
 the beta1 receptor
 02:53
 and that's how it prevents an increase
 02:56
 in the heart rate
 02:58
 in a system that has fair receptors
 03:00
 maximum efficacy is reached
 03:02
 even before receptor occupancy is
 03:04
 hundred percent
 03:06
 an antagonist can inhibit the actions
 03:09
 of all types of agonists
 03:12
 to learn more about the action of
 03:14
 antagonists
 03:15
 let's talk a bit about what is efficacy
 03:18
 potency
 03:19
 and what are spare receptors efficacy
 03:23
 is the maximal or hundred percent
 03:25
 response that is produced
 03:27
 after a drug binds to a receptor whereas
 03:31
 potency is the dose or amount of drug
 03:34
 that is needed
 03:35
 to produce a given response it is
 03:37
 determined by
 03:38
 affinity of the drug to its receptor and
 03:41
 also
 03:42
 by the number of receptors that are
 03:44
 available
 03:46
 both efficacy and potency can be plotted
 03:49
 on a dose response curve
 03:51
 what are spare receptors spare receptors
 03:55
 are receptors that are present in excess
 03:59
 of the number of receptors that are
 04:01
 needed to produce
 04:02
 a maximum response antagonist can be
 04:06
 competitive
 04:07
 non-competitive or irreversible
 04:11
 a competitive antagonist binds to the
 04:13
 same receptor as the agonist
 04:17
 it decreases the potency of the drug
 04:20
 but it does not change the efficacy of
 04:22
 the drug
 04:24
 the action of a competitive antagonist
 04:26
 can be reversed
 04:28
 by just increasing the dose of the
 04:30
 agonist
 04:31
 an example will be fromazanile which is
 04:34
 an antagonist
 04:35
 at the gaba receptor for that reason
 04:38
 flumezanil
 04:39
 is used as an antidote in the treatment
 04:42
 of
 04:43
 benzodiazepine poisoning on the other
 04:46
 hand
 04:46
 a non-competitive antagonist binds to a
 04:49
 different
 04:50
 site than that bound by an agonist
 04:54
 it decreases the efficacy of a drug but
 04:57
 does not change the potency of a drug
 05:00
 the effect of a non-competitive
 05:02
 antagonist
 05:04
 cannot be reversed by just increasing
 05:06
 the dose of the agonist
 05:08
 an example would be ketamine
 05:11
 which is a non-competitive antagonist at
 05:14
 the
 05:14
 nmda receptor an irreversible antagonist
 05:18
 binds to the receptor with high affinity
 05:21
 and then it cannot be displaced
 05:23
 it behaves like a non-competitive
 05:25
 antagonist
 05:26
 in the sense that it decreases the
 05:29
 efficacy
 05:30
 but does not change the potency but
 05:33
 if spare receptors are present then an
 05:37
 irreversible antagonist will decrease
 05:39
 the efficacy
 05:40
 as well as the potency so to recap
 05:44
 an agonist binds at a receptor to
 05:47
 produce the desired response
 05:49
 a partial agonist also activates the
 05:51
 receptor
 05:52
 but produces less than the maximal
 05:55
 response
 05:56
 an inverse agonist binds to the receptor
 06:00
 and produces an effect that is opposite
 06:03
 to the effect of a full agonist
 06:05
 an antagonist does not have any inherent
 06:09
 activity on the receptor it just
 06:12
 prevents
 06:12
 binding of the agonist on the receptor
 06:16
 a competitive antagonist decreases the
 06:19
 potency of a drug
 06:20
 without changing the efficacy of a drug
 06:23
 while
 06:24
 a non-competitive antagonist and
 06:26
 irreversible antagonist
 06:28
 will decrease the efficacy of a drug
 06:31
 without changing the potency of a drug
 06:33
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