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Henderson-‐Hasselbalch • The difference between the pH of the solu7on and the pKa of the drug is the common logarithm of the ra7o of ionized to unionized forms of the drug.
• For acid drugs Log(Ionized/Unionized) = pH -‐ pKa, or [I]/[U] = 10(pH-‐pKa) Deriva'on:
Ka = [H+][A-‐]/[HA] -‐log Ka = -‐log([H+][A-‐]/[HA]) -‐log Ka = -‐log[H+] -‐ log ([A-‐]/[HA]) pKa = pH -‐ log ([A-‐]/[HA]) log ([A-‐]/[HA]) = pH -‐pKa
H.H.: a quan7ta7ve picture
• Most drugs are weak acids or weak bases
• It is not all or nothing, there are always several species at different concentra7ons pKapH
BHB
pKapHHAA
−=⎟⎟⎠
⎞⎜⎜⎝
⎛
+
−=⎟⎟⎠
⎞⎜⎜⎝
⎛ −
][][log
][][log
Frac7on Ionized as a func7on of pKa and pH
+ -‐
0 0
Weak acid is mostly neutral in stomach
A drug is a weak acid, has a pKa of 5.5. Taken orally, it is in a stomach solu7on of pH 3.5.
pH – pKa = 3.5 – 5.5 = -‐2 For an acid, we use: ionized/unionized = 10-‐2/1= 1/100
For every 1 molecule of the drug that is ionized, 100 are unionized. This drug in the stomach is highly fat soluble.
Basic Drugs For basic drugs, everything is the
same except that the ra#o reverses:
Log(Unionized/Ionized) = pH – pKa
Examples: Chlorpheniramine, chlorpromazine, ephedrine and phenylephrine, amphetamine, methamphetamine, and methcathinone, amitriptyline, imipramine, lofepramine and clomipramine, nortriptyline, desipramine, and amoxapine.
pKapHBHB
pKapHHAA
−=⎟⎟⎠
⎞⎜⎜⎝
⎛
+
−=⎟⎟⎠
⎞⎜⎜⎝
⎛ −
][][log
][][log
NH2+ [Cl-‐]
Amphoteric drugs
• Ordinary ampholytes, e.g. m-‐aminophenol • pKaacidic > pKabasic. pKaA=9.8, pKaB=4.4 • Increasing pH: 1. NH3+ 2. Neutral 3. O-‐
Zwicerionic ampholytes
• E.g. Amino acids, pep7des
• pKaacidic < pKabasic
Distribu7on of ionic species for the zwicerionic ampholyte labetalol. from A. Pagliara, P.-‐A. Carrupt, G. Caron, P. Gaillard and B. Testa, Chem. Rev., 97, 3385 (1997).
Isoelectric Point • pI = pH = ½ ( pKaacidic + pKa basic ) • It can be both uncharged and zwicerionic or mul7ply charged
• For absorp7on every charge counts (not the total charge)
• pI is used for isoelectric focusing (Agarose gel electrophoresis)
Zwicerion drugs: Examples
Calcula7on of the pH of drug solu7ons
• The drug solu7on itself can develop its own pH
• The pH can be derived from its pKa and concentra7on, C
A weakly acidic drug: HA + H2O=[A-‐] + [H30+] (1-‐a)c ac ac a – degree of dissocia7on
a<<1 ac=[H+] at c >≈ 10-‐7M
Note: this approxima#on
breaks at infinitesimal concentra#ons.
cpKpH a log21
21 −=
Ka =a2c2
(1− a)c≈ a2c
a2 = Ka / ca2c2 = Kac
[H+]= (Kac)12
− log([H+]) = − log((Kac)12 )
− log([H+]) = − log(Ka )− log(c)12
Deriva'on:
For the solu'on of a weakly acidic drug:
Weakly basic drugs • Similarly it can be shown that
• Example: codeine monohydrate (317.4), pKa=8.2
C=0.026 M pH=7+4.1-‐0.79
cpKpHcpKpKpH
a
aw
log7log
21
21
21
21
21
++=
++=
Basic Amine is charged at neutral pH
Ion Trapping of an acidic drug
The same highly fat soluble drug readily crosses the stomach membranes and enters blood plasma, which has a pH of 7.5
pH – pKa = 7.5 – 5.5 = 2 [I]/[U] = 102/1= 100/1 For every 100 molecules of the drug that are ionized, only 1 is unionized. The drug in the blood is not very fat soluble.
This phenomenon is called ion trapping.
Absorp7on is quan7ta7ve too
• Permea7on rate, RP= P Area ΔC • Amount absorbed = RP• Time
– P is membrane par77on coefficient (related to LogP)
– Area is effec7ve surface area of membrane
– ΔC is concentra7on difference for the neutral form of the drug
• Astomach/Aileum differ 1000 7mes. That means that even if the frac7on of neutral species in stomach is 100 7mes greater, s7ll 10 7mes more compound will be absorbed in the gut. The effect of 7me comes on top.
Cout
Cin
A=120m2
A=0.1m2
Drug Salts: Bases
Note: Hydrochloride in Drug.HCl may be misleading, it should be just chloride (Drug+.[Cl-‐]), same with H2SO4
Acid [AH]
Basic Drug [B] + -‐
Drug, BH+ Anion
Anionic Salt of the Drug
plus
Acid Anion Examples
Hydrochloride (HCl) Cl-‐ Pyridoxine HCl, Pramipexole HCl Chlorpromazine HCl, Demeclocycline .. Demethylchlortetracycline Nalbuphine, Chlorhexidine Propafenone, Mitoxantrone Lincomycin, Ro7go7ne Vilazodone, Naphazoline
Sulfuric Acid SO42-‐ Dextroamphetamine, Hydroxychloroquine
Ace7c Acid (acetate) CH3COO-‐ Leuprolide, Goserelin, Desmopressin,…
cocaine hydrochloride
+
Drug Salts: Acids Base B, e.g. NaOH
Acidic Drug AH
e.g. R-‐COOH -‐ + Drug
ion, [A-‐] Ca7on
Ca7onic Salt of the Drug
plus
OH-‐ Base Ca'on Examples
Sodium Na+ Ecabet, Diclofenac, Indomethacin, Benzoate, Salicylate
Calcium Ca++ Atorvasta7n, Calcium Gluceptate
Potassium K+ Penicillin V Potassium, Losartan
Ca++ needs two nega7ve charges
Calcium Gluceptate
Warning: do not forget to use correct molecular weight of the salt
Salts: summary • Stoichiometry: make sure
that the total formal charge is zero (e.g. [D-‐]2 Me2+ )
• Ambiguity of chemical representa7on: [DH+][Cl-‐] vs [D][HCL], MolWeight.
• Solubility of crystals: usually becer than non-‐salt, but differs between different salts.
• Iden#cal in Solu#on: Once the salt is dissolved it becomes iden7cal to the non-‐salt form of the drug in solu7on
• Resonance: Example, sulfate: perfect tetrahedron with total nega7ve charge of -‐2 distributed between 4 nega7ve oxygens and one posi7ves sulfur.
Salts with becer solubility: example • Example:
– Phenobarbital, a white powder, is a weak acid with limited solubility in water,
– the sodium salt of Phenobarbital, also a white powder, the salt of the weak acid, now water soluble
• pKa = 7.41
• pH (satur. sol) 5 ~ 10. • Solubility: 1g/L 1g/10mL
Epoprostenol.Na+ Prostacyclin I.V. vasodilator in ischemia & PH
Na+
More examples: Naproxen Naproxen Sodium Fenoprofen Fenoprofen Calcium Penicillin G Penicillin G Potassium
More anions for basic drugs • Base Salt/Conjugate Acid • Diphenhydramine Diphenhydramine HCL • Glucosamine Glucosamine sulfate • Epinephrine Epinephrine sulfate • Ephedrine Ephedrine HCl • Atropine Atropine sulfate • Tetracycline Tetracycline HCl
• Most of these drugs, as you can tell by their name, are "amines", which means they are weak bases
• Acetate CH3COO− (ace7c acid) • Carbonate CO3
2− ,carbonic acid) • Chloride Cl− (hydrochloric acid) • Citrate HOC(COO−)(CH2COO−)2 (citric acid) • Cyanide C≡N− (hydrocyanic acid) (toxic) • Nitrate NO3
− (nitric acid) • Nitrite NO2
− (nitrous acid) • Phosphate PO4
3− (phosphoric acid) • Sulfate SO4
2− (sulfuric acid) Sodium-‐nitroprusside, -‐ vasodilator
Solubility and Permeability
Two opposite requirements: • Solubility is good for charged compounds with mul7ple polar groups (e.g. pep7des)
• Permeability is good for hydrophobic and apolar compounds
Two solu7ons: • Be exactly in the middle with the same chemical structure (minority)
• Be able to change via enzyma7c ac7va7on (prodrug) or adopt alterna7ve charged forms
Solubility, LogP, and LogD
• Not all uncharged compounds are insoluble
• Not all polar or charged compounds can not permeate a membrane
• It is a quan#ta#ve macer • Three measurable quan77es are used to characterize a drug substance: LogSw, LogP and LogD
water
membrane
Drug Solubility: defini7on • Water (aqueous) solubility (SW) is the maximum amount of a substance that can dissolve in water. SW depends on P,T.
• Sw is in moles/L (M). Watch for mg/L or mg/dL !
• LogSw (or LogS ) = Log(Sw)
Sucrose
Succinylcholine >10M! But not fat soluble Mitotane: 0.1 mg/L
Solubility and Gibbs energy
• Solubility is defined by a difference between the free energy in the crystal form (primarily enthalpy) and the dissolved form (solva7on, different entropy terms)
• The entropy-‐of-‐mixing contribu7on to dissolu7on (and rigid body rota7on/transla7on) does not depend on chemical type and interac7ons. The main difference: – the number of freed rotatable bonds, hydrophobic surface, (ΔS);
– intermolecular interac7ons in the crystal vs water, ΔH
water
µ 0aq + RT lnSW = µ 0crystal
Polymorphism • Compounds can crystallize as different polymorphs (different molecular conforma7on and packing, cell)
• Polymorphs can have drama7cally different solubility, mel7ng point, 7me of dissolu7on, habits
A
AA
AA
AAA
A
AA
A
A
A
AA
AA
AAA
A
AA
A
A
A
A
A
A
A
AA
A A
AA A
A AA
AAA A
AA A AA
A
A
A
A
A
AA
A A
AA A
A AA
AAA A
AA A AA
G
AA
AAA
AAA
G
G
G
GGG
A
A
A
GG
AG
G
AA
AAA
AAA
G
G
G
GGG
A
A
A
GG
AG
C-A+
C- C-C-
C- C-C-C-
A+
A+
A+A+
A+
A+A+
A+C-
A+C- C-C-
C- C-C-C-
A+
A+
A+A+
A+
A+A+
A+
Salts
Co-‐crystals??
Polymorphs Same API Same Ac've Moiety
Different API
Where Do Co-‐Crystals Fit?
Is a New Regulatory Class of Solids Needed?
Adopted from the presenta7on of FDA-‐Div-‐Director Dr. Andre S. Raw
API: Ac7ve Pharmaceu7cal Ingridients
Crystal habits of drugs • The same symmetry group may lead to
different size and shape of a crystal • Crystal habits (and size) may influence
– injec7on (plates: easy, needles: difficult), – tablexng (easy for compression) – rate of dissolu7on
• Habits: – Acicular (needle-‐like) – Prisma7c, pyramidal, tabular, equant,
columnar an lamellar types • Habit determinants:
– Solvent – Temperature – Concentra7on of impuri7es
Snow flakes & drug crystals
• Snow flakes are a very well known example, where subtle differences in crystal growth condi7ons result in different geometries.
Par7cle size and the rate of dissolu7on
• Consider the surface of the fixed amount of compound as the func7on of linear micro-‐crystal size, d, and the total volume V
• For non-‐cubic shapes, calculate the Area as a func7on of total volume and shape.
dVd
dVA 66 23 ==
d
For simple cubic shape the total area of microcrystal surface is inversely propor7onal to the crystal size
Log D and Membrane permea7on
• To get inside the cell a drug need to get inside the membrane first
• Par##oning between water and a membrane is characterized by LogP for hard drugs and LogD for ionizable drugs
• Nega7ve LogD: too polar • Large posi7ve LogD: – too hydrophobic
The quan7ta7ve octanol/water model. Molecule Does not Change: LogP
wat
oct
CCP loglog =
OH
O
OH
Owater octanol
• P means Par77on • Octanol ~ membrane • Free energy difference
HOH
RTCCP
CRTCRT
ow
w
o
ooww
3.2loglog
lnln00
00
µµ
µµ
−==
+=+
oo
ww
CC,
,0
0
µ
µ drug in water drug in octanol
Benzoic Acid: LogP = 1.87
The octanol/water model: LogD
wat
oct
AHAHP][][loglog =
OH
O
OH
Owater octanol
watwat
octoct
AAHAAHD][][][][loglog
−
−
+
+=
O
O
O
O
LogD is the apparent par77on coefficient
LogD depends on LogP of the neutral form and pH-‐pKa
wat
oct
AHAHP][][loglog =
OH
O
OH
O
logD = logP - log(1 + 10pH-pKa) for acids ≈ logP – (pH – pKa) (for pH> pKa+1, charged form dominates)
water octanol
watwat
octoct
AAHAAHD][][][][loglog
−
−
+
+=
O
O
O
O
logD = logP - log(1 + 10-(pH-pKa)) for bases ≈ logP + (pH – pKa) (for pH < pKa-1, charged form dominates)
LogD is apparent LogP
pKa = 4.2; LogD (pH=7.2) ≈ 1.87-‐ 3 = -‐1.13
Our body is watching the lipophilicity of xenobio7cs
Kidneys take care of the polar Liver takes care of the hydrophobs
• Polar and charged molecules – renal clearance (fast). *Probenecid (OAT inh.) increases excre7on of uric acid but blocks renal excre7on of and other drugs.
• BBB blocks the polar and charged compounds
• Hydrophobic compounds are made into polar ones by metabolism. Cyp450s modify hydrophobic compounds,
Lipophilicity – a determinant of pharmacokine7cs
• LogD, pH=7.4 (from Shalaeva, ““New Technologies to Increase Drug Candidate Survivability”, Philadelphia, 2002)
• < 0 Too polar. Intes7nal and CNS permeability problems. Suscep7ble to renal clearence
• 0 to 1 A good balance between permeability and solubility. At low values, (more polar), CNS permeability may suffer
• 1 to 3 Op'mum range for CNS and non-‐CNS orally bioavailable drugs. Low metabolic liabili'es, generally good CNS penetra'on
• 3 to 5 Solubility tends to become lower. Metabolic liabili7es.
• Above 5 Low solubility and poor oral bioavailability. Erra7c absorp7on. High metabolic liability, although potency may s7ll be high.
0
1
2
3
4
5
LogD
Cytochrome P450 • R-‐H + O2 + 2e => R-‐OH + H2O (uses NADPH) • Adding One Oxygen: monooxygenase • R-‐OH is further modified by solubilizing sulfate or sugars • bergamoxn, dihydroxybergamoxn, and paradicin-‐A in
grape fruit juice (and other juices) have been found to inhibit CYP3A4 , -‐ overdose
• Saint-‐John's wort induces CYP3A4, but also inhibits CYP1A1, CYP1B1, and CYP2D6, -‐ no ac7on
• Tobacco smoking induces CYP1A2, ..
4-‐hydroxy-‐tamoxifen in the estrogen receptor pocket
Cytochrome P450 2B4 with paroxe7ne
Problema7c permeability
• Natural products (big and polar) – permeability a major problem
• Pep7domime7cs (long and polar) – permeability a major problem
• RNAi • CNS targets (7ght barrier) – Blood-‐brain permeability a major problem
Solubility/Permeability gate • Permeability – PSA > 140-‐200 Å2 is problema7c for systemic distr. – PSA > 75 Å2 is problema7c for CNS delivery
• Solubility – Solubility < 5-‐20 µg/mL is problema7c
• Poor permeability is worse than poor solubility -‐ no easy formula#on fix exists
• Intra-‐molecular H-‐bonds improve permeability with minimal affect on solubility.
Pro-‐drugs to improve solubility
NH
Cl
OO
O-Na+O
O
ClOH
O2N
NH
Cl
OH
O
ClOH
O2N
O-Na+O
O
OH
Esterase
or Water
Chloramphenicol Succinate
Chloramphenicol
Sodium succinate
Drug OPromoiety
OOH Promoiety
Promoiety ODrug
O
Drug OH
O
Promoiety OH
OOH Drug
or
+
+ Enapril +H2O
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