2. FARMACOCINETICA E FARMACODINAMICA :
IMPLICAZIONI CLINICHE
Giovanni Severino
Universita degli Studi di Cagliari,
Department of Biomedical Sciences
Italy
5. Assorbimento
Enterale: orale sublinguale rettale
Parenterale: endovenosa intramuscolare subcutanea
intrarteriosa intracardiaca
Vie d’organo: intrarticolare inalatoria oculare
•inalazione di un broncolitico
•infiltrazione di un glucocorticoide nelle
articolazioni
6.
7. L’entità dell’assorbimento di un farmaco per via orale
dipende dal suo pKa, dalla sua lipofilia e dal pH del mezzo
Assorbimento di un farmaco per via orale
[farmaco] nella fase oleosa
COEFFICIENTE DI RIPARTIZIONE = -----------------------------------
[farmaco] nella fase acquosa
farmaci sono acidi o basi deboli (PKa)
il coefficiente varia a seconda del pH
non è un parametro fisso
PH < PKa deprotonato PH > PKa protonato
8. Razionale per la scelta della via di somministrazione
Vie di somministrazione di un farmaco
Tipo di farmaco
degradati nello stomaco
metabolizzati (inattivati)
non assorbiti o distribuiti
Patologia
acuzie vs. trattamento cronico
stato del paziente (incoscienza o coma)
9. Via Orale
• Assorbimento irregolare
• “Onset” dell’effetto terapeutico variabile
e distante nel tempo
• Metabolismo epatico elevato
(inattivazione) prima della distribuzione
• Inutilizzabile nelle urgenze
• Facilità della somministrazione
• Compliance del paziente
Via Endovenosa
• Costo e difficoltà della
somministrazione
• Scarsa compliance del paziente
• Effetti tossici improvvisi
• Infezioni
• Embolismo
• Dosaggio preciso
• “Onset” dell’effetto terapeutico rapido
• Possibile in ogni stato patologico
• Possibile per ogni tipo e quantità
(grandi) di farmaco
VANTAGGI
SVANTAGGI
Via Orale Via Endovenosa
10. Proteine
Grosse molecole
Fagocitosi
pinocitosi
Glucosio
Acqua
Urea
Alcool
Filtrazione
attraverso i pori
Trasporto
attivo
Sostanze idrosolubili
non ionizzabili
diametro molecolare
> 4 <4
ATP
ADP-Pi
Diffusione
facilitata
Acidi e basi
organiche ionizzate
Diffusione
semplice
Sostanze liposolubili
Elettroliti deboli
Doppio
strato
lipidico
caratteristiche chimico-fisiche
Passaggio delle molecole attraverso le
membrane biologiche
11. Elettroliti deboli
(la maggior parte dei farmaci)
Sostanze idrosolubili non ionizzate con
Grande diametro molecolare
(glucosio)
Diffusione facilitata
Trasportatore
Acidi e basi organiche ionizzate
Proteine e, in generale, grosse molecole Fagocitosi e pinocitosi
(Vescicole)
Caratteristiche del farmaco Passaggio attraverso le membrane
Sostanze idrosolubili, ionizzate, con
Piccolo diametro molecolare
(acqua, urea, alcool)
Filtrazione
Diffusione semplice
Trasporto attivo
Trasportatore - energia
pori
Passaggio delle molecole (dei farmaci)
attraverso le membrane biologiche
12. pH gastricopH 2
Intestino pH 5-7
Dissoluzione
Via di Somministrazione orale vs endovenosa
Solubilizzazione.
Assorbimentoorale
Circolazione
Sistemica
Circolo Portale
Escrezione biliare,
Via
endovenosa
Metabolismo
13. Biodisponibilità
ConcentrazionePlasmatica
Tempo
F i.v. = AUC A = 100%
AUC A
AUC B
F assoluta della via orale=
Curve Concentrazione -Tempo
via endovenosa = A
B
via orale = B
Stesso
farmaco
A
a frazione della dose somministrata che
raggiunge la circolazione sistemica
la velocità con cui ciò avviene
14. Il tempo di dimezzamento o tempo di emivita
E’ il tempo necessario affinché la concentrazione [C] plasmatica si
riduca della metà dopo un’unica somministrazione del farmaco
Reazioni di ordine zero la velocità della reazione non dipende dalla
[C] del substrato
Pertanto t1/2 = [A]o / 2k
e quindi il tempo di
dimezzamento dipende dalla
concentrazione iniziale del
reagente.
15. Il tempo di dimezzamento o tempo di emivita
E’ il tempo necessario affinché la concentrazione [C] plasmatica si
riduca della metà dopo un’unica somministrazione del farmaco
Reazione di primo ordine : il tempo di dimezzamento dipende solo
dalla costante di velocità e non dalla concentrazione iniziale
t1/2 = ln 2/k = 0.693/k
In una reazione del primo
ordine quindi il tempo di
dimezzamento dipende solo
dalla costante di velocità e
non dalla concentrazione
iniziale dei reagenti
16. Somministrazioni ripetute
Somministrando un farmaco a intervalli di una emivita si ottengono
minime oscillazione della concentrazione ematica intorno a quella
terapeutica (stady-state)
Sono necessarie circa 5-7 emivite per raggiungere lo stady-state
Sono necessarie almeno 7 emivite per ottenere il wash-out del farmaco
N.B. in alcuni casi l’effetto farmacologico può durare più a lungo (es. farmaci con meccanismi irreversibili, farmaci che
interferiscono con la sintesi di proteine…..
17. Tempo di latenza Tempo
Picco ematico
Inizio eff. terapeutici
Fase di eliminazione
Fasediassorbimento
[C]
Concentrazione minima tossica
Fine eff. terapeutico
Durata d’azione
Range Terapeutico
L’intervallo di concentrazioni ematiche di un farmaco entro il quale si manifestano
normalmente gli effetti terapeutici senza effetti tossici dose-dipendenti
30. ● Classe Molecolare:Classe Molecolare: Enzima (Idrolasi)Enzima (Idrolasi)
● Funzione Molecolare:Funzione Molecolare: Attività CataliticaAttività Catalitica
● Processo Biologico:Processo Biologico: Metabolismo EnergeticoMetabolismo Energetico
● Peso Molecolare:Peso Molecolare: 55948 Da55948 Da
● Locus Genico:Locus Genico: 10q24.1-q24.310q24.1-q24.3
● Simbolo Genico:Simbolo Genico: CYP2C19CYP2C19
● Espressione Genica:Espressione Genica: Fegato, Piccolo Intestino, Mucosa NasaleFegato, Piccolo Intestino, Mucosa Nasale
● Localizzazione Subcellulare:Localizzazione Subcellulare: Reticolo EndoplasmaticoReticolo Endoplasmatico
● Segnale Peptidico:Segnale Peptidico: 1-26 aa1-26 aa
● Aminoacidi:Aminoacidi: 490 aa490 aa
● Nomi Alternativi:Nomi Alternativi: Monossigenasi legata alla Flavoproteina,Monossigenasi legata alla Flavoproteina,
CYP2C19, P450C2C, Mefenitoina 4' Idrolasi, EC1.14.13.80,CYP2C19, P450C2C, Mefenitoina 4' Idrolasi, EC1.14.13.80,
EC1.14.13.48, EC1.14.13.49, CPCJ, CYP2C, CYP 2C,EC1.14.13.48, EC1.14.13.49, CPCJ, CYP2C, CYP 2C,
P450C2C, P450IIC19P450C2C, P450IIC19
Citocromo P450, sottofamiglia 2C, polipeptide 19
CARATTERISTICHE
31. Metabolismo Epatico
fase I metabolismo
(CYP P450) ossido-
riduzione
Fase II Reazioni di
biosintesi
Genetica
Comparto
Plasmatico
Effetto di 1°
passaggio
Fase I Fase II
32. • Betabloccanti: Labetalolo, Metoprololo Propranololo
• Calcio-antagonisti: Diltiazem Felodipine Isradipine,
Nicardipine, Nifedipine Nimodipine, Nitrendipine,
Verapamil
• Farmaci psichiatrici: Desipramine, Doxepin,
Imipramine, Sulpiride
• Sedativi/Ipnotici: benzodiazepine
• Oppiacei/Antagonisti: Morfina, Naltrexon
Farmaci con importante effettoFarmaci con importante effetto
di 1° passaggio
34. CYP 1A2CYP 1A2
CYP2C19CYP2C19
CYP 2E1CYP 2E1
ruolo significativo
per alcune
classi di farmaci
CYP3A4CYP3A4
CYP2DCYP2D
66
CYP2CCYP2C
99
metabolizzano il
80 – 90 % dei
farmaci
CYP clinicamente più importanti
44. quota libera da
farmaci di piccole
dimensioni e
idrofili viene
Fenomeno di
riassorbimento:
influenzato dal pH
urinario e pKa
meccanismi di
trasporto
per gli acidi (ASA,
penicillina)
per le basi
(morfina).
trasporto mediato
da proteinetasso di filtrazione glomerulare
46. Farmacodinamica
EFFETTO
FARMACOLOGIC
O
Cosa il farmaco fa al corpo
ORGANI BERSAGLIO
RECETTORI
Farmaco libero disponibile per l’azione
Complesso
Farmaco Recettore
Le molecole di un farmaco devono essere legate a
particolari costituenti cellulari e tissutali (con qualche
eccezione) “un farmaco non funziona se non si lega”
Ehrlich 1854-1915).
47. Nessun farmaco é completamente specifico
nella sua azione
In molti casi l’aumento della dose di un farmaco,
provocherà l’interazione con altri bersagli diversi da
quello primario, e porterà alla comparsa di effetti
indesiderati di cui nessun farmaco é privo.
Esempio: Farmaci Antistaminici
bersaglio primario: ridurre la reazione allergica legata
all’aumento dell’istamina endogena
effetto indesiderato la sonnolenza.
48. Definizioni
Recettore:
elemento in grado di legarsi ad un altro elemento e di
iniziare una risposta biologica
Ligando :
una molecola che si lega ed è riconosciuta da un
recettore
Effettore:
l’elemento che inizia una risposta biologica
49. Recettore
Affinchè una proteina possa essere classificata come recettore (e non una semplice
proteina di legame) devono essere soddisfatti diversi criteri
Specificità – un recettore deve essere in grado di distinguere tra segnali spesso
strettamente correlati
Alta affinità – I segnali sono spesso presenti a basse concentrazioni – recettori
efficienti possono spesso captare concentrazioni tra nM e pM
Saturabilità – una cellula ha un numero finito di recettori, quindi vi è un limite
al numero di molecole di ligando che una cellula può legare
Reversibilità – l’associazione ligando-recettore non è covalente – quando la
concentrazione del ligando diminuisce il complesso può
dissociarsi
Accoppiamento – il recettore trasferisce un segnale dal ligando alla cellula
50. Cosa fanno I segnali
Le cellule rispondono ai segnali in una varietà di modi
Alterazione del metabolismo p.es. Alterazione del metabolismo del
glicogeno in risposta all’insulina
Eccitamento p.es. propagazione dell’impulso nervoso in risposta a
neurotrasmettitori
Crescita e Divisione (mitosi) in risposta a fattori di crescita
Morte cellulare programmata causata da specifici fattori di ‘morte’
o dalla rimozione di alcuni fattori essenziali
Espressione genica alterata – p.es. Sintesi di immunoglobuline in
risposta a citochine
51. La via facile del Signalling
Se esiste un segnale permeabile attraverso la membrana plasmatica
e che è prodotto in (relativa) abbondanza, allora il signalling è
particolarmente semplice….
Nucleo
In questo esempio il
cortisolo (uno steroide)
può attraversare la
membrana legandosi al
recettore per i
glucocorticoidi (GR).
Tale recettore è anche un
fattore di trascrizione e
legando il cortisolo migra
nel nucleo e attgiva la
trascrizione di specifici
geni.
Cortisolo
GR
52. Far attraversare il segnale
Molti segnali (p.es. fattori di crescita) non
possono attraversare la membrana
plasmatica. Le cellule risolvono questo
problema utilizzando recettori
transmembrana. Questi sono proteine
posizionate in membrana con il sito attivo
per il ligando* sul lato extracellulare e un
dominio intracellulare che si accoppia allo
step successivo nella catena di trasduzione
del segnale
Dominio di
legame
extracellulare
Dominio
intracellulare – si
accoppia allo step
successivo (può avere
attività enzimatica)
Dominio
transmembrana
Membrana plasmatica
esterno
iinterno
*
53. Piccole proteine monomeriche in grado di attivare (S) o inattivare (i) altre
proteine
α
GDP +Pi GTPGTP
ATTIVA
INATTIVA
Scambio del GDP
legato col GTP
La subunità α attiva può
interagire con lo step
successivo della catena
Dissociazione Attività GTPasica
della subunità α
GTP GDP+Pi
Le proteine G
α
γ
GDP
β
α
Effettore
54. Proteine di fosforilazione
Interruttori On-Off
Protein Kinasi – trasferiscono un
fosfato dall’ATP ad amino acidi
specifici
C
CCO
O
H
NH
Serina
C
CCO
O
NH
OP
O-
O-
O-fosfoserina
ChinasiATP
ADP
Protein Fosfatasi – rimuovono un
fosfato da specifici amino acidi
C
CCO
O
H
NH
Serina
C
CCO
O
NH
OP
O-
O-
Fosfatasi
Pi
Fosforilazione Defosforilazione
O-fosfoserina
55. β1-RecettoreAdenilato Ciclasi
AMPc ATP
Proteine Fosforilate
Cambio di attività di :
Enzimi, canali,
Fattori di trascrizione
Gproteina
α γ
β
Interazione Ligando-Proteina
Reazione enzimatica
Protein Kinasi A
Interazione Ligando-Proteina
ATP
ADP
Proteina P
Interazione Proteina-Proteina
Chimica del segnale
Β agonista
56. Nucleo
Utilizzo di Secondi Messaggeri
β γ α
P P
2Pi
ATP
ATP
ADP
La subunità
regolatrice della
PKA lega l’AMPc…
…e si dissocia dalla
subunità catalitica
La subunità libera
della PKA migra nel
nucleo
La subunità catalitica della
PKA fosforila CREB e attiva
la trascrizione
Protein chinasi A (PKA)
inattiva
Il binding dell’
adrenalina al suo
recettore attiva una G
proteina
GTP
CREB
Adenilato Ciclasi
AMP ciclico (AMPc)
57. PLC
DAG
Calcio – il messaggero universale
kinasi
L’IP3 si lega al
recettore per l’IP3
sul reticolo
endoplasmatico e
apre un canale del
Ca2+
(che fa parte
del recettore)
L’attivazione della PLC
porta alla formazione di IP3
solubile in acqua
Il Ca2+
rilasciato dal RE si lega alle
Calmoduline permettendole di interagire
con altre proteine e attivarle
La Calmodulina può attivare pompe del
Ca2+
del reticolo endoplasmatico
abbassando la [Ca2+
] citosolico
La Calmodulina può
attivare pompe del
Ca2+
sulla membrana
plasmatica abbassando
la [Ca2+
] citosolico
La Calmodulina attiva una vasta gamma di
proteine p.es chinasi calmodulina-dipendenti
PKC
Ca2
Calmoduline
pompe del Ca2
α
γ βα
IP3
58. Amplificazione
1 recettore attiva
più proteine G
Ciascun enzima X
produce molti secondi
messaggeri, ciascun
messaggero attiva 1
enzima Y
AMPLIFICAZIONE
AMPLIFICAZIONE
AMPLIFICAZIONE
AMPLIFICAZIONE
AMPc
Kinasi A
Enzima Z
Prodotti dell’
enzima Z
proteina recettore
una molecola di
ligando
subunità α
di Gs
Adenilato ciclasi
attivata
Ciascuna kinasi A può
fosforilare e attivare molte
copie di enzima Z
Ciascuna copia di enzima Z
produce molte molecole di
prodotto
1
recettore-ligando
500
Proteine G
500
enzimi
105
2ndi
messaggeri
250
canali ionici
105
-107
ioni
Una molecola di
rodopsina assorbe un
fotone
59. Potenza : misura di quanto farmaco è necessario per evocare
una data risposta. Più è bassa la dose richiesta, più il
farmaco è potente. Dipende dall’affinità del
farmaco per il recettore. La potenza si esprime come
la dose che provoca il 50% della risposta ED50
Efficacia : È la risposta massima prodotta da un farmaco e
dipende dal numero di complessi farmaco-recettore
formati e dall’efficienza con cui il recettore attivato
produce un’azione cellulare
COMPLESSO
Farmaco - Recettore
61. Attività di una molecola agonista
1 10 100 1000 10000 1000000
0
25
50
75
100
AgonistaA
AgonistaB
AgonistaC
AgonistaD
%Effetto
Agonista parziale B
Agonista pieno
Agonista parziale A
Agonista pieno
Agonista Log Dose
62. Analgesia
La potenza da sola è un vantaggio che non deve essere
sopravvalutato in terapia.
Se il farmaco A è piu’ potente del farmaco B, ma non possiede
altri vantaggi particolari, cio’ significa soltanto che il paziente
potra’ assumere dosi minori.
afficacia
Dose
Efficacia o Attività Intrinseca
effetto massimo di un farmaco
(aspirina e morfina producono lo stesso
effetto farmacologico [analgesia] ma hanno
livelli di efficacia molto diversi)
64. Antagonista non competitivo
AA Antagonista
0
25
50
75
100
%Effetto
Agonista Log Dose
Agonista A da solo
AA +Antagonista conc 1
AA +Antagonista conc 10
AA +Antagonista conc 100
66. UN FARMACO MOLTO POTENTE PUÒ NON
RAGGIUNGERE IL SITO D’AZIONELA SUA EFFICACIA
SARÀ BASSA RISPETTO AD UN FARMACO MENO
POTENTE CHE RAGGIUNGE PIU’ FACILMENTE IL SITO
D’AZIONE
67. 100
Effettobiologico
50
0
ED 50 del
farmaco A in presenza
di un antagonista competitivo
ED 50 del
farmaco A da solo
concentrazione del farmaco
ED 50 del
farmaco A
in presenza
di un
agonista
parziale
Antagonista competitivo
sposta verso destra la
curva dose rispostapotenza
Comportamento di un farmaco (A) in presenza di altri farmaci
Interazione farmacodinamica
68. Diversa efficacia di un farmaco
Stessi sintomi
Stessa malattia
Stesse
manifestazioni
Stesso farmaco
Diversi effetti
?
Differenze genetiche
Cause:
•Interazioni tra farmaci
•Caso
•Oppure……………
69. La variabilità genetica influisce dal
20 al 95% nella differenza inter-
individuale riscontrata dopo
l’assunzione di un farmaco
73. - CYP3A4: responsabile dell’80 %
del metabolismo della codeina
(inibito da voriconazolo e
claritromicina)
- CYP2D6: ultrarapid metabolism
responsabile del 10% del
metabolismo di codeina
-Insufficienza renale acuta per
accumulo glucuronidi
75% della codeina totale viene
trasformato in morfina e
rispettivi metaboliti rispetto al
10% dell’individuo normale
Esempio di polimorfismo a livello di citocromo P450
Gasche Y et al., NEJM, 2004, 351: 2827-31
74. Evans et al., NEJM 2003; 348: 538-49
Influenza della
Variabilità
genetica della
p-Gp sulla
farmacocinetica
Digossina
CC TT
CCTT CT
NEFENAVIR C-T polimorfismo
G-T polimorfismo
Cambio di nucleotide
nell’esone 21 e di
funzione
Esone 26
CC normo funz
CT iperfunz
TT ipofunz
fexofenadina
fexofenadina
La fexofenadina cloridrato è un anti-
istaminico anti H1 non sedativo
Ma !!! Sovradosaggio =
vertigini, sonnolenza, affaticamento
e secchezza delle fauci
75. Gene β2AR: recettore β2adrenergico contiene un
polimorfismo (Arg16Gly)(Arg16Gly) responsabile di una diversa
risposta all’albuterolo adrenergico betastimolante
broncodilatatore
Recettore β2adrenergico
76. Arg16Gly
• diminuisce la sintesi di nuovo recettore
• Modifica la risposta al trattamento con beta
antagonisti
Genotipo Risposta
Arg/Arg 5,3
Arg/Gly
2,3
Gly/Gly
1
Risposta albuterolo
volume espiratorio
forzato
adrenergico betastimolante
77. Meccanismo dell’induzione o
dell’inibizione dell’attività enzimatica
Diminuzione di attività Aumento di attività
Ormoni:
Reprimono la trascrizione
genica legandosi ai recettori
che agiscono come fattori
trascrizionali negativi attivati
da ligando
Xenobiotici induttori:
Attivano la trascrizione genica
legandosi ai recettori che
agiscono come fattori
trascrizionali positivi attivati
da ligando
Geni
Trascrizione
Traduzione
Enzimi
Inibitori enzimatici:
agiscono direttamente
sull’enzima per inibirne
l’attività
Fattori attivanti:
Agiscono direttamente
sull’enzima stimolandone
l’attività
78. PotenziamentoPotenziamento
effetti sedativieffetti sedativi
Farmaci ad attività
deprimente il SNC
Interazioni farmacodinamiche dei triciclici
Farmaci ad
attività
anticolinergica
Farmaci
cardiovascolari
+
+
+
Triciclici
PotenziamentoPotenziamento
effettieffetti
anticolinergicianticolinergici
PotenziamentoPotenziamento
effetto ipotensivo eeffetto ipotensivo e
rischio di aritmierischio di aritmie
80. Interazioni tra farmaci
Addizione
Farmaco A Farmaco B
Meccanismo comune
Effetto finale
=
Effetto (A+B)
Sommazione
Farmaco A Farmaco B
Meccanismo A Meccanismo B
Effetto finale
=
Effetto A + Effetto B
Sulfametossazolo
Sulfadimetossina
Streptomicina-Penicillina
82. Interazioni tra farmaci
Antagonismo
Farmaco A Farmaco B
Recettore A Recettore B
Effetti opposti
a quelli di A
Effetto finale < effetto A
• Antagonismo recettoriale
Farmaco A Farmaco B
(agonista) (antagonista)
Recettore
Effetto finale < effetto A
83. il CYP2C9 si occupa di S Warfarin
il CYP3A4 si occupa di R Warfarin
VKORK1: gene codificante per la vitamina K-
epossido reduttasi complesso 1 che
rappresenta il target del farmaco
variazioni della sequenza del genedella sequenza del gene
sono correlati con diverse espressionisono correlati con diverse espressioni
della proteina (mRNA) a livellodella proteina (mRNA) a livello
epaticoepatico
CYP2C9*1 - normale
CYP2C9*2 intermedio
CYP2C9*3 basso
86. Pompelmi, arance amare, I componenti del succo di pompelmo e di altri
agrumi sono infatti in grado di INIBIRE l'attività del CYP3A4
I prodotti della pirolisi della carne e pesce (derivati di AA tipo
TRIPTOFANO) sono degli INDUTTORI; come pure derivati INDOLICI
presenti nelle verdure.
SUCCO DI POMPELMOSUCCO DI POMPELMO
FARMACI POTENZIALMENTE A RISCHIO DI
INTERAZIONE CON IL SUCCO DI POMPELMO
AtorvastatinaAtorvastatina
CiclosporinaCiclosporina
SirolimusSirolimus
NifedipinaNifedipina
VerapamilVerapamil
rischio miopatia, rabdomiolisirischio miopatia, rabdomiolisi
tossicità renaletossicità renale
tossicità ematologicatossicità ematologica
ipotensione grave, ischemia miocardicaipotensione grave, ischemia miocardica
flushing, edema, ischemia miocardicaflushing, edema, ischemia miocardica
87. L’INFLUENZA DI GENI E AMBIENTE
NELLA RISPOSTA AI FARMACI
- Geni
- Ambiente
RISPOSTA
AL FARMACO
MALATTIE
CONCOMITANTI
ETA’ E
FUNZIONALITA’
EPATICA
ADERENZA
ALLA TERAPIA
DOSAGGIO
INTERAZIONI
CON ALTRI
FARMACI
PATOGENESI
E GRAVITA’
DELLA
MALATTIA
GENI
DELL’ASSORBIMENTO,
DISTRIBUZIONE,
METABOLISMO, ED
ELIMINAZIONE
GENI DEL
BERSAGLIO DEL
FARMACO
ALTRI GENI
Editor's Notes
Il tratto GI, al pari di altre membrane, si comporta come una barriera lipofila
Acidi e basi sono assorbiti di preferenza in forma indissociata
La maggior parte dei farmaci è assorbita per diffusione passiva
La velocità di assorbimento e la quantità di farmaco assorbita sono correlate al coefficiente di ripartizione
Acidi deboli possono essere assorbiti nello stomaco, ma non le basi deboli
Il metabolismo ha luogo principalmente nel fegato.
Alcuni farmaci raggiungono il fegato, tramite la vena porta, subito dopo essere stati assorbiti (nella parte superiore dell&apos;intestino) e vengono estesamente metabolizzati prima di raggiungere la circolazione sistemica. Questo fenomeno, noto come metabolismo di primo passaggio, fa sì che il farmaco venga inattivato prima di raggiungere la circolazione sistemica e gli organi bersaglio: eliminazione pre-sistemica. L&apos;eliminazione pre-sistemica può essere dovuta anche al metabolismo a livello della mucosa intestinale.
Nelle reazioni di ordine zero la velocità della reazione non dipende dalla [C] del substrato : riportando in grafico sull’asse delle ascisse il tempo e su quella delle ordinate la concentrazione si ottiene una retta con pendenza negativa e si nota che il tempo di dimezzamento diminuisce quando la concentrazione diminuisce.
Nelle reazioni di ordine zero la velocità della reazione non dipende dalla [C] del substrato : riportando in grafico sull’asse delle ascisse il tempo e su quella delle ordinate la concentrazione si ottiene una retta con pendenza negativa e si nota che il tempo di dimezzamento diminuisce quando la concentrazione diminuisce.
Dopo aver raggiunto il circolo ematico, i farmaci si distribuiscono nei vari tessuti; il processo prosegue fino al raggiungimento di un &quot;equilibrio di distribuzione&quot; nel quale le concentrazioni tissutali e plasmatiche si equivalgono.In condizioni di equilibrio, ogni variazione delle concentrazioni plasmatiche si riflette in una modificazione delle concentrazioni tissutali. La relazione fra concentrazione plasmatica e tissutale è espressa appunto dal volume di distribuzione, Vd , detto anche volume di distribuzione apparente, per sottolineare che non è un&apos;entità anatomica, ma un volume teorico che indica come il farmaco si dovrebbe distribuire per avere una concentrazione uguale a quella del plasma, qualora la distribuzione fosse monocompartimentale.
Il metabolismo ha luogo principalmente nel fegato.
Alcuni farmaci raggiungono il fegato, tramite la vena porta, subito dopo essere stati assorbiti (nella parte superiore dell&apos;intestino) e vengono estesamente metabolizzati prima di raggiungere la circolazione sistemica. Questo fenomeno, noto come metabolismo di primo passaggio, fa sì che il farmaco venga inattivato prima di raggiungere la circolazione sistemica e gli organi bersaglio: eliminazione pre-sistemica. L&apos;eliminazione pre-sistemica può essere dovuta anche al metabolismo a livello della mucosa intestinale.
General Pharmacologic Principles
I. INTRODUCTION. Pharmacology is the study of the biochemical and physiologic aspects of drug effects, including absorption, distribution, metabolism, elimination, toxicity, and specific mechanisms of drug action. The two main areas of pharmacology are pharmacokinetics and pharmacodynamics.
A. Pharmacokinetics refers to the way the body handles drug absorption, distribution, biotransformation, and excretion. Once the pharmacokinetics of a drug is determined, rational dosage regimens can be instituted.
B. Pharmacodynamics is the study of the biochemical and physiologic effects of drugs and their mechanisms of action.
II. PHARMACOKINETICS
A. General principles
1. Drug transport. The movement of drug molecules in the body affects absorption, distribution, and excretion. Drugs can cross cellular membranes by simple diffusion, carrier-mediated diffusion, filtration, active transport, or endocytosis. The cell membrane, being a bimolecular lipid layer, can also act as a barrier to some drugs.
a. Passive diffusion. Most foreign compounds penetrate cells by diffusing as the un-ionized moiety through the lipid membrane. Factors affecting the passage of a molecule through a membrane are the molecule&apos;s size and charge, the lipid-water partition coefficient, and the concentration gradient. The two types of passive drug transport are simple diffusion and filtration.
(1) Simple diffusion
(a) Simple diffusion is based on Fick&apos;s law:
dQ/dt = (- D)(A)(dc/dx)
where dQ/dt is the rate of drug flux (i.e., the change in concentration of a drug within a given time); D is a temperature-dependent diffusion constant of the molecule; A is the area of the absorbing surface; and dc/dx is the concentration gradient.
(i) The greater the concentration gradient, the greater the rate of absorption.
(ii) The larger the absorbing surface, the greater the drug flux.
(iii) The diffusion constant, D, is directly proportional to the temperature and is inversely related to the molecular size.
(iv) The greater the lipid-water partition coefficient, the greater the drug flux.
(b) In simple diffusion, molecules cross the lipid membrane in an uncharged form. The distribution of the uncharged form is a function of the pKa of the compound and the pH of the medium and is expressed by the Henderson-Hasselbalch equation:
(i) If the drug is a weak acid:
K = H + 10 concentration of un-ionized acid
Pa P g .
f&apos; &apos;
d&apos;
dconcentration 0 Ionize aci
(ii) If the drug is a weak base:
K = H + 10 concentration of ionized base
PaP g
t.f&apos; ,
dbconcen ration 0 un-Ionize ase
(c) The pH of the medium, therefore, affects the absorption and excretion of a passively diffused drug.
(i) Aspirin and other weak acids are best absorbed in the stomach because of its acidic environment.
(ii) Alkalinicdrugsare best absorbed in the small intestine, which has a higher pH.
(iii) Since the pH of urine is acidic, a weakly acidic drug can be extensively re-absorbed into the body from the urine. If the pH of the urine is increased, excretion of the drug can be increased.
(2) Filtration
(a) Water, ions, and some polar and nonpolar molecules of low molecular weight can diffuse through membranes, suggesting that pores or channels may exist.
(b) The capillaries of some vascular beds (e.g., in the kidney) have large pores, which permit the passage of molecules as large as proteins.
b. Carrier-mediated facilitated diffusion
(1) In this type of transport, movement across the membrane is facilitated by a macro-molecule.
(2) The properties of carrier-mediated diffusion are as follows:
(a) It is a saturable process; that is, external concentrations can be achieved in which increasing the external/internal concentration gradient will not increase the rate of influx.
(b) It is selective for the chemical structure of a drug; that is, the carrier mechanism transports only those drugs having a specific molecular configuration.
(c) It requires no energy.
(d) It cannot move against a concentration gradient and, therefore, is still a diffusion process.
c. Active transport
(1) Active transport is similar to carrier-mediated diffusion in several ways:
(a) Movement across the membrane is mediated by a macromolecule.
(b) It is a saturable process.
(c) It is selective for chemical structure.
(2) Several important features, however, distinguish active transport from diffusion processes:
(a) Active transport requires metabolic energy; this is often generated by the enzyme known as Na+-K+-ATPase.
(b) It transports molecules against a concentration gradient. .
d. Endocytosis is a minor method by which some drugs are transported into cells.
(1) A vacuolar apparatus in some cells is responsible for this process.
(2) There exist both fluid-phase endocytosis for substances such as sucrose and adsorptive-phase endocytosis for substances such as insulin.
2. Bioavailability is the relative rate and extent to which an administered drug reaches the general circulation; this is especially important when a drug is administered orally. Factors that influence bioavailability are:
a. Solubility of the drug in the contents of the stomach &apos;:&apos;
1&quot;; b. Dietary patterns I~
c. Tablet size r
d. Quality control in manufacturing and formulation c,
B. Absorption is the rate at which a drug leaves the site of administration and the extent to which this occurs. The absorption of a drug through the mucosal lining of the gastrointestinal tract or through capillary walls depends on the physical and chemical properties of the drug.
1. Route of administration is an important determinant of the rate and efficiency of absorption.
a. Alimentary routes are the most common routes of administration.
(1) Examples of alimentary routes
(a) Oral
(b) Rectal
(c) Sublingual
(d) Buccal
(2) Advantages of alimentary administration ..
(a) An alimentary route is generally the safest route of administration. The delivery of the drug into the circulation is slow after oral administration, so that rapid, high
(b) The dosage forms available for alimentary administration are convenient and do not require sterile technique.
(3) Disadvantages of alimentary administration
(a) The rate of absorption varies. This becomes a problem if a small range in blood levels separates a drug&apos;s desired therapeutic effect from its toxic effects.
(b) Irritation of mucosal surfaces can occur.
(c) Patient compliance is not ensured.
(d) With oral administration of some drugs, extensive hepatic metabolism may occur before the drug reaches its site of action. This is known as the first-pass effect.
Passage through the liver and the resulting initial hepatic metabolism are avoided by administering the drug sublingually.
b. Parenteral routes bypass the alimentary tract.
(1) Examples of parenteral routes
(a) Intravenous
(b) Intramuscular
(c) Subcutaneous
(d) Intraperitoneal
(e) Intra-arterial
(f) Intrathecal
(g) Transdermal
(2) Advantages of parenteral administration
(a) The drug gets to the site of action more quickly, providing a rapid response, which may be required in an emergency.
(b) The dose can often be more accurately delivered.
(c) Parenteral administration can be used when the alimentary route is not feasible (e.g., when the patient is unconscious).
(d) Large volumes can be delivered intravenously.
(3) Disadvantages of parenteral administration
(a) More rapid absorption can lead to increased adverse effects.
(b) A sterile formulation and an aseptic technique are required.
(c) Local irritation may occur at the site of injection.
(d) Parenteral administration is not suitable for insoluble substances.
c. Miscellaneous routes
(1) Topical administration is useful in the treatment of patients with local conditions; with topical administration, there is usually little systemic absorption. Drugs can be applied to various mucous membranes and skin.
(2) Inhalation provides a rapid access to circulation; it is the common route of administration for gaseous and volatile drugs.
2. Factors affecting drug absorption
a. Solubility of the drug affects absorption.
b. Dosage affects the drug&apos;s concentration at its site of action and, thus, greatly influences the biologic response to the drug. The larger the dose, the greater the effect, until a maximum effect is achieved. This is called the dose-response relationship.
c. Route of administration. The route of administration affects the area of absorbing surface available to the drug. Drugs are absorbed more quickly from large surface areas.
(1) After any route of administration except intravenous administration, the absorption of most drugs follows first-order (exponential) kinetics; thus, a constant fraction of drug is absorbed.
(2) After intravenous administration, the absorption of a drug follows zero-order kinetics; thus, a constant amount (i.e., 100%) of drug is absorbed.
C. Distribution. After absorption or injection, drugs may be distributed into interstitial or cellular fluids.
1. Drug distribution
a. Once in the circulatory system, some drugs can bind nonspecifically and reversibly to various plasma proteins; that is, to albumin or globulins.
(1) In this case, the bound and free drug reach an equilibrium.
(2) Only the free drug exerts a biologic effect; the bound drug stays in the vascular space, and is not metabolized or eliminated.
b. Some areas of the body (e.g., the brain) are not readily accessible to drugs due to anatomic barriers. The placenta also provides a barrier to some drugs.
c. Some drugs may be sequestered in storage depots; for example, lipid-soluble drugs in fatty tissue. The drug stored in these depots is in equilibrium with free circulating drug.
d. Eventually, the drug achieves a free state and is excreted either directly or after it has been metabolized.
e. Factors modifying the distribution of a drug to a particular region of the body:
(1) Physical and chemical characteristics of the drug (lipid to water partition coefficient)
(2) Cardiac output
(3) Capillary permeability in various tissues
(4) Lipid content of the tissue
(5) Binding to plasma proteins and tissues
2. Clinical distribution
a. One-compartment model. This is the simplest and most commonly used pharmacokinetic model system.
(1) Distribution of the drug within the compartment is assumed to be uniform and is assumed to occur rapidly in comparison to absorption and elimination.
(2) The apparent volume of distribution (V d) is a quantitative estimate of the tissue localization of the drug.
(a) It can be determined by measuring the plasma level of the drug: iii,
,,&apos;
V_total amount of drug in body !;C,:
d-
concentration of drug in plasma
(b) In general, a high V d indicates high lipophilicity or many receptors for the drug.
(3) The total body clearance is the volume of blood or plasma that is effectively cleared of drug in a specified unit of time.
(a) Clearance is related to V d and to the time required for the plasma drug concentration or the amount in the body to decrease by 50%, which is called half-life
(t1/2)&apos; Clearance is, therefore, related to the elimination rate constant (k):
Clearance = Vd(k)
= Vd(O.693)
t1/2
(b) This formula assumes a specific Vd, but the Vd changes over time.
b. Two-compartment model
(1) This model is generally used for drugs that are not administered intravenously because it can better describe both distribution and elimination.
(2) The distribution rate constant in this model is known as the alpha half-life, or t1/2a&apos;
(3) The elimination rate constant in this model, known as the beta half-life, or t1/2P&apos; is therapeutically more important than the t1/2a&apos;
c. The multicompartment model is used for drugs that are stored in body depots and for drugs with extensive metabolism or elimination mechanisms.
D. Drug metabolism/biotransformation is the process of chemical alteration of drugs in the body.
a. The liver is the major site of metabolism for many drugs or other xenobiotics, but other organs, such as the lungs, kidneys, and adrenal glands, can also metabolize drugs.
b. Many lipid-soluble, weak organic acids or bases are not readily eliminated from the body and must be conjugated or metabolized to compounds that are more polar and less lipid-soluble before being excreted.
c. Metabolism often, but not always, results in inactivation of the compound.
d. Some drugs are activated by metabolism. These substances are called prodrugs.
2. Biochemical reactions involved in drug metabolism occur in two phases: Phase 1 reactions (e.g., oxidation, reduction, hydrolysis) alter chemical reactivity and increase aqueous solubility; phase 2 reactions (e.g., conjugation) further increase the solubility, promoting elimination.
a. Oxidation, the most common metabolic reaction, involves the addition of oxygen or the removal of hydrogen from the drug.
(1) Microsomal oxidation
(a) The smooth endoplasmic reticulum of cells in many organs, especially the liver, contain membrane-associated enzymes, which are responsible for drug oxidation.
(b) The subcellular components of the endoplasmic reticulum, called microsomes, can be isolated by centrifugation of organ homogenates.
(c) The primary components of this enzyme system are cytochrome P-450 reductase and cytochrome P-450. Several distinct isozymes of cytochrome P-450 exist within the microsomal membrane.
(d) This enzyme system has been termed a mixed-function oxygenase because one oxygen atom in molecular oxygen is incorporated into a drug in the form of a hydroxyl (-OH) moiety, and the other is incorporated into water. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) provides the reducing equivalents.
(e) Types of microsomal oxidation reactions are:
(i) Carbon oxidation-hydroxylation of aliphatic or aromatic groups (e.g., pentobarbital and phenytoin)
(ii) N- or O-dealkylation (e.g., diazepam and codeine)
(iii) N-oxidation or N-hydroxylation (e.g., 2-acetylaminofluorene)
(iv) Sulfoxide formulation (e.g., chlorpromazine)
(v) Deamination (e.g., amphetamine)
(vi) Desulfuration (e.g., thiobarbital)
(f) The microsomal oxidative system also metabolizes endogenous fatty acids and steroids.
(g) A number of drugs and environmental substances can induce (increase) or inhibit the microsomal enzyme system (see Chapter 13 II 0 5).
(2) Nonmicrosomal oxidation
(a) Soluble enzymes found in the cytosol or mitochondria of cells are responsible for the metabolism of relatively few compounds. These enzyme activities are, however, important.
(b) Examples include:
(i) Alcohol dehydrogenase and aldehyde dehydrogenase, which oxidize ethanol to acetaldehyde and acetate, respectively, in reactions requiring oxidized nicotinamide adenine dinucleotide (NAD+)
(ii) Xanthine oxidase, which converts hypoxanthine to xanthine a~d xanthine to uric acid
(iii) Tyrosine hydroxylase, which is important in the synthesis of adrenergic neurotransmitters because it hydroxylates tyrosine to dopa (L-~-3,4-dihydroxy-phenylala.,ine)
(iv) Monoamine oxidase, which is important to the metabolism of catecholamines and serotonin
b. Reduction occurs in both the microsomal and nonmicrosomal metabolizing systems; it is less common than oxidation.
(1) Examples of microsomal reduction include nitro (chloramphenicol) and azo (prontosil).
(2) Examples of nonmicrosomal reduction include aldehyde (chloral hydrate), ketone (naloxone), and quinone (menadione).
c. Hydrolysis
(1) Nonmicrosomal hydrolases exist in a variety of body systems, including the plasma.
Examples of nonmicrosomal hydrolases are:
(a) Nonspecific esterases for drugs, such as acetylcholine, succinylcholine, and procaine
(b) Peptidases (e.g., proinsulin)
(c) Amidases for drugs, such as procainamide and indomethacin
(2) Microsomal hydrolases have also been identified.
d. Conjugation involves the coupling of a drug or its metabolite with an endogenous substrate, usually inorganic sulfate, a methyl group, acetic acid, an amino acid, or a carbohydrate. Usually, a conjugate is a readily excreted polar substance.
(1) Glucuronide conjugation is the most common conjugation reaction. It occurs frequently with phenols, alcohols, and carboxylic acids.
(a) Activated glucuronic acid (uridine diphosphoglucuronic acid; UDP-glucuronide) is formed from glucose 1-phosphate, which then reacts as follows:
UDP-glucuronide + ROH -+ RO-glucuronide + UDP where ROH is the drug or its metabolite.
(b) Glucuronides are generally inactive and are rapidly excreted in urine or bile by anionic transport systems.
(c) Glucuronides eliminated in the bile can be hydrolyzed by intestinal or bacterial l3-glucuronidases, and free drug can be reabsorbed. This enterohepatic recirculation can greatly extend the action of a drug.
(2) Other conjugation reactions. All conjugations except glucuronide formation are catalyzed by nonmicrosomal enzymes. These reactions include:
(a) Sulfate formation, in which phenols, alcohols, and aromatic amines are converted to sulfates and sulfanilates; 3&apos;-phosphoadenosine 5&apos;-phosphosulfate (PAPS) is the sulfate donor (e.g., steroids)
(b) 0-,5-, and N-methylation; 5-adenosylmethionine is the methyl donor(e.g., norepinephrine)
(c) N-acetylation; acetyl coenzyme A is the acetyl donor (e.g., isoniazid)
(d) Glycine and glutamine conjugation with acids (e.g., salicylic acid)
(e) Glutathione conjugation (e.g., ethacrynic acid)
3. Factors affecting drug metabolism
a. Genetics. The most important factor is genetically determined polymorph isms. The acetylation of isoniazid and the hydrolysis of succinylcholine are genetically controlled.
b. Chemical properties of the drug. Certain drugs may stimulate or inhibit the metabolism of the other drugs (e.g., phenobarbital stimulates the metabolism of diphenylhydantoin).
c. Route of administration. The oral route, for example, can result in extensive hepatic metabolism of some drugs (the first-pass effect).
d. Diet. Starvation can deplete glycine stores and alter glycine conjugation.
e. Dosage. Toxic doses can deplete enzymes needed for detoxification reactions.
f. Age. The liver cannot detoxify drugs, such as chloramphenicol, as well in neonates as it can in adults.
g. Gender. Young males are more prone to sedation from barbiturates than females.
h. Disease. Liver disease decreases the ability to metabolize drugs, while kidney disease hampers the excretion of drugs.
i. Species differences. Experimental findings in animalsdo not necessarily translate to humans.
j. Circadian rhythm. In rats and mice, the rate of hepatic metabolism of some drugs follows a diurnal rhythm. This may be true in humans as well.
E. Drug excretion is the process by which a drug or metabolite is eliminated from the body.
1. Rate of elimination
a. For most drugs, elimination from the blood follows exponential (first-order) kinetics.
b. The elimination process can be saturated after high doses of some drugs, and elimination will then follow zero-order kinetics. Ethanol is a prototypic example.
c. For drugs that are eliminated by first-order kinetics, the fractional change in the amount of drug in the plasma or blood per unit of time is expressed by the half-life (t1/z)-or by the elimination rate constant (k), which is equal to O.693/tl/z.
2. Routes of elimination
a. The kidney is the most important organ for excretion of drugs. Excretion of drugs and their metabolites into the urine involves three processes:
(1) Glomerular filtration. Water-soluble and polar compounds are unable to diffuse back into circulation and are excreted unless a specific transport exists for their reabsorption.
(2) Active tubular secretion. Mechanisms for active tubular secretion exist in the proximal tubule. Drugs such as organic acids (e.g., penicillin, bases, quinine) are transported by these systems.
(3) Passive tubular reabsorption
b. The biliary tract and the feces are important routes of excretion for some drugs that are metabolized in the liver.
c. Other routes. Drugs and their metabolites can also be eliminated in expired air, sweat, saliva, tears, and breast milk. Drugs eliminated through these routes tend to be lipid-soluble and non ionized.
F. Effect of repeated doses
1. A drug accumulates in the body if the time interval between doses is less than four of its half- lives, in which case the total body stores of the drug increase exponentially to a plateau. This plateau is known as the steady-state concentration.
General Pharmacologic Principles 7
2. The average total body store of a drug at the plateau is a function of the dose, the interval between doses, and the elimination half-life of the drug.
a. When the drug is administered in intervals that equal its half-life of elimination, the average total body store of the drug is approximately equal to 1.5 times the amount administered.
b. When the drug is given by constant infusion, the plasma concentration will be within 10% of the desired steady-state concentration after four elimination half-lives.
III. PHARMACODYNAMICS
A. Mechanisms of drug action
1. It is currently believed that most drugs interact with macromolecular components (called receptors) of a cell or an organism to begin biochemical and physiologic changes that produce the drug&apos;s observed effects or response.
a. Receptors bind ligands and transduce signals.
b. Drug receptors are identified primarily by the effect or lack of effect of antagonists and the relative strength of agonists.
(1) A drug is called an agonist if it produces some of the effects of endogenous compounds when it interacts with the receptor. The agonist, acetylcholine, has intrinsic activity.
(2) An antagonist is a drug that has no intrinsic activity yet can reduce or abolish the effect of an agonist.
(a) Examples of pure antagonists are atropine and curare, which inhibit the effect of acetylcholine.
(b) Atropine and curare occupy cholinergic receptor sites (curare in nicotinic skeletal muscle, atropine in muscarinic smooth muscle; see Chapter 2 I D), preventing the further binding of acetylcholine to these receptors.
(3) Some drugs (e.g., nalorphine) are partial antagonists; that is, they possess some intrinsic activity.
c. In general, the effect (E) is a function of the quantity of the drug-receptor complex (DR),
and can be expressed as:
E = a[DR]
(1) The magnitude of E depends on the amount of DR, which, in turn, depends on the amount of drug given. Once all receptors are saturated, the maximum effect is achieved.
(2) Alpha (a) is a constant for a given drug and is a partial determinant of whether an effect occurs.
(3) Alpha (a) is determined experimentally and is considered to be a measure of a drug&apos;s intrinsic activity; that is, its inherent ability to produce an effect. If a is a zero, the drug has no intrinsic pharmacologic activity, and, therefore, if the drug binds to the receptor, it must be an effective antagonist.
2. Other drugs may not cause a response by interacting with receptors. These drugs may combine with small molecules or ions found in the body (e.g., chelating agents).
B. Receptors are specific drug-binding sites in a cell or on its surface, which mediate the action of the drug. Some drugs (e.g., mannitol) are believed not to have specific receptors.
1. Ion channel
a. Hormones and neurotransmitters enhance the movement of ions across membranes.
b. The hormone or neurotransmitter receptor may be an ion channel.
c. Ion receptors are confined to excitable tissue (e.g., central nervous system, neuromuscular junction, autonomic ganglia).
d. Agonists that activate ion channel receptors produce depolarization or hyperpolarization.
e. Examples of ion channel receptors include the nicotinic acetylcholine receptor, the gamma-
aminobutyric acid (GABA) receptor, the glutamine receptor, and the glycine receptor.
2. G proteins
a. Receptors on the inner face of the plasma membrane regulate or facilitate effector proteins through a group of guanosine triphosphate (GTP) proteins known as G proteins.
b. Some hormone peptide receptors and neurotransmitter receptors (e.g., a and ~ adrenergic and muscarinic receptors) depend on G proteins to mediate their actions on cells.
c. Each G protein can respond to several different receptors and regulate several effectors.
d. Effectors include adenylyl cyclase and phospholipases C and A2 and channels specific for Na+, K+, and Ca2+.
e. Each cell can express several G proteins. Each G protein consists of a, ~, and &apos;Y subunits. Specificity for receptor-effector coupling lies within the a portion.
f. Termination of signal transmission results from hydrolysis of GTP to guanosine diphosphate
(GOP) by GTPase that is intrinsic to the a subunit.
3. Hormones
a. Steroid hormones bind to intracellular receptors. These receptors are proteins associated with the nuclear matrix.
b. The steroid receptor complex ultimately increases binding of RNA polymerase and the expression of regulated genes.
c. Examples of such receptors include those for estrogen, progesterone, glucocorticoid, tri-iodothyronine (T 3)&apos; thyroxine (T 4), and vitamin O.
4. Tyrosine kinases
a. A variety of growth factors and growth-promoting hormones [e.g., colony-stimulating factor (CSF), epidermal growth factor (EGF), insulin-life growth factor (IGF-1)1 and certain oncogenes interact with receptors that have tyrosine-specific protein kinase activity.
b. These receptors are located on the plasma membrane and are critical proteins in cell growth and differentiation.
C. Receptor regulation
1. Certain receptors when exposed to an agonist repeatedly can become desensitized or down- regulated. For example, ~-adrenergic bronchodilators used in the treatment of patients with asthma can become less effective over time when administered at the same concentration. The signal from the stimulated receptor can become depressed, or the receptor itself can be altered.
2. Supersensitivity of receptors to agonists can OCCur with chronic administration of an antagonist. For instance, the abrupt discontinuation of propranolol in a patient who had been taking it chronically, could precipitate dysrhythmias. Supersensitivity may result from the synthesis of additional receptors (up-regulation).
D. Dose-response relationships
1. Quantal dose-response curve
a. A quantal response is an all-or-none response to a drug and relates to the frequency with which a specified dose of a drug produces a specific response in a population (e.g., death among the mice in a preclinical study or a 20% decrease in blood pressure among the patients in a clinical trial).
b. The smallest amount of a drug that will produce a quantal response is not the same for all members of a population. If the frequency of response is plotted against the minimum dose necessary to produce the response, the result is often a gaussian distribution. A graphic representation of this gaussian frequency distribution curve is shown in Figure 1-1.
c. Thequantal dose-response curve is a cumulative graph of the frequency distribution curve
(Figure 1-2).
2. Graded dose-response curve (Figure 1-3)
a. As the dosage administered to a single subject or isolated tissue is increased, the pharmacologic effect will also increase (graded dose-response).
b. At a certain dose, the resulting effect will reach a maximum level (ceiling effect).
Figure 1-1. Frequency distribution curve, plotting the
number of patients showing a quanta&apos; response to a drug
I against the minimum dose needed to produce the re-
(mgikg) sponse.
Figure 1-2. Quantal dose-response curve, cumulating
Dose the data used in plotting Figure 1-1.
c. Efficacy versus potency
(1) Efficacy is the maximum effect of a drug, Emax (see Figure 1-3).
(a) Efficacy is independent of the slope or position of the dose-response curve. In Figure 1-3, drugs A and B have equal efficacies (i.e., the same Emax).
(b) Drugs such as aspirin and morphine produce the same pharmacologic effect (analgesia) but have very different levels of efficacy.
(2) Potency, a comparative measure, refers to the different doses of two drugs that are needed to produce the same effect.
(a) In Figure 1-3, drug A is more potent than drug B, since the dose of drug B (DB) must be larger than the dose of drug A (DA) to produce a given effect (E1).
(b) Meaningful comparisons of potency can be made only when the drugs being compared have log dose-response curves (see III D 3) of the same slope.
(c) KD (the dissociation constant for the drug-receptor complex) and the EDso [see III D 3 a (1)] are measures of drug potency.
(3) Potency is independent of efficacy, and efficacy is usually more important than potency in selecting drugs for clinical use.
3. Log dose-response curve
a. To construct the log-dose response curve, the log of the dose is plotted on the abscissa and the drug effect on the ordinate. This causes the hyperbolic graded dose-response curve to become sigmoidal. Log dose-response curves for two drugs, A and B, are shown in Figure 1&apos;:4.
(1) The EDso, the smallest dose showing an effect that is 50% of the maximum, is a measure of drug potency: The smaller the EDso, the greater the potency. Thus, in Figure 1-4, drug A is more potent than drug B.
(2) Efficacy is indicated by the height of the log dose-response curve. Drug B in Figure 1-4 is less efficacious than drug A; the Emax of the two drugs is, therefore, different.
b. One advantage of plotting on a logarithmic scale is that drugs with the same action at a receptor, but with different potencies, usually will show dose-response curves that are parallel.
4. Antagonism between drugs
a. Pharmacologic antagonism occurs when an antagonist prevents an agonist from interacting with its receptors to produce an effect. This type of antagonism can be either competitive or noncompetitive.
Figure 1.3. Graded dose-response curves for two drugs,
D D A and B. Emax = maximum effect; DA and DB = amount
A B (dose) of drug A and drug B, respectively, needed to pro-
Dose duce the drug effect, E,.
Log dose showing an effect that is 50% of the Emax.
(1) Competitive antagonism
(a) Competitive antagonists compete with agonists in a reversible fashion for the same receptor site.
(b) When the antagonist is present, the log dose-response curve is shifted to the right, indicating that a higher concentration of agonist is necessary to achieve the same response as when the antagonist is absent.
(c) In the presence of the antagonist, if enough agonist is given, the Emax can be achieved, indicating that the action of the antagonist has been overcome. This results in a parallel shift of the dose-response curve, as shown in Figure 1-5.
(2) Noncompetitive antagonism
(a) The noncompetitive antagonist binds irreversibly to the receptor site or to another site that inhibits the response to the agonist.
(b) No matter how much agonist is given, the action of the antagonist cannot be overcome.
(c) This results in a nonparallel shift of the log dose-response curve with a lower Emax (Figure 1-6).
b. Physiologic antagonism. With pharmacologic antagonism, the agonist and antagonist compete for the same receptor site. In contrast, with physiologic antagonism, the drugs act independently on two different receptors.
(1) Physiologic antagonism is exemplified by one drug acting on the sympathetic nervous system causing the heart rate to increase and causing vasoconstriction; while another drug acting on the parasympathetic nervous system decreases the heart rate and causes vasodilation.
(2) Using physiologic antagonists is generally less preferable than using a receptor- specific antagonist. Since physiologic antagonists work on different receptors, they produce effects that are harder to control than the effects of a receptor-specific antagonist.
c. Antagonism by neutralization (also called chemical antagonism) occurs when two drugs combine with one another to form an inactive compound. For example, drugs containing sulfhydryl (-SH) groups, when combined with mercury or arsenic, no longer show agonistic actions.
Figure 1-5. Shift in the log dose-response curve that oc-
curs when an agonist is administered in the presence of
og ose a competitive antagonist.
Figure 1-6. Shift in the log dose-response curve and lowering of the maximum effect (Emax) that occur when an agonist is given in the presence of a noncompetitive antagonist.
5. Enhancement of drug effects
a. Additive drug effects occur if two drugs with the same effect, when given together, produce an effect that is equal in magnitude to the sum of the effects when the drugs are given individually:
EAB = EA + EB 1 + 1 = 2
b. Synergism occurs if two drugs with the same effect, when given together, produce an effect that is greater in magnitude than the sum of the effects when the drugs are given individually:
EAB &gt; EA + EB 1 + 1 &gt; 2
c. Potentiation occur$ if a drug lacking an effect of its own increases the effect of a second, active drug:
EAB &gt; EA + EB 0 + 1 &gt; 1
6. Therapeutic index (TI) and margin of safety
a. The therapeutic index is a ratio used to evaluate the safety and usefulness of a drug for an indication. It is a measurement that describes the relationship between doses of a drug required to produce undesired and desired effects.
(1) The formula for the therapeutic index is:
LDso
TI =-
EDso
where LDso is the mimimum dose that is lethal or toxic for 50% of the population, and
EDso is the minimum dose that is effective for 50% of the population.*
(2) Ideally I the LDso should be a much higher dose than the EDso, so that the therapeutic index would be large.
b. Standard margin of safety. The theraPf:!utic index may be misleading if the log dose-response curves for effectiveness and toxicity have different slopes (i.e., are not parallel).
Therefore, the standard margin of safety may be more useful.
(1) The formula for the standard margin of safety is:
(LD)
1 -1 x 100
(ED99)
(2) The standard margin of safety shows the percentage by which the ED99 (the dose effective in 99% of the population) must be increased to cause toxic effects in 1 % of the population.
(3) For example, if 100 mg of a drug causes toxicity in 1 % of the population and 10 mg is effective in 99%, then the drug&apos;s standard margin of safety is (100/1 0) -1 x 100, or 900. Therefore, the dose that is effective in 99% must be increased 900% to be toxic to 1 % of the population.
Note that the definition for EDso here is for a population rather than a single individual or tissue, and thus it differs from the EDso in III D 3 a (1).
*Esempio di antagonismo chimico= warfarin (acido) neutralizzato da protamina solfato (base)
