PHYSICAL-CHEMICAL CHARACTERIZATION of API

PHYSICAL-CHEMICAL
CHEMICAL
CHARACTERIZATION
of API
E.F.”Gene
Gene” Fiese, Ph. D.
Pharmaceutics Consultant
Ledyard, CT 06339

Initial Objective
• Does this molecule possess physical properties
adequate for development
• What’s s Broken
• Want to know in advance
– Projected human dose – order of magnitude
– Route of administration – oral or IV
– Desired Dosage Form

Initial Bulk Caveat
Impure
Amorphous or crystalline mix
Undefined Salt or Polymorph Form
Meaningless Data!!!!!

Solubility Analysis
• pH Solubility Profile: 1-9, 1
RT
• pKa: two methods
• Solubility and pH in water, PBS & 5% dextrose
• Non Aqueous Solvents
• Solubilization with 20% ß-cyclodextrin
• Micellar Solubilization with 5% Tween 80
• Identify IV Toxicology Vehicle

Reality Check
Water Solubility
6.5 = Human Dose/250 ml
S6.5
– 1 ug/ml = 0.25 mg human dose
– 1 mg/ml = 250 mg human dose
– Your API

Influence of Ionization pKa
• Only Un-Ionized Species Gets
Absorbed Across Membranes
• Ionized Species Most Water Soluble
• Pump or Carrier Mediated
Transport of Ionic Species

Examples of pKa measurement
• Titration
• pH - solubility
• UV specta shift
• Rt by RP-HPLC
• NMR
• Always
extrapolate to
100% aqueous

Oxindoles

Tenidap UV Spectra f(pH)

Henderson-Hasselbalch
Hasselbalch Eq.
Weak Acid
HA = H +
= A -
pH = pKa + log [salt/acid]
pH = pKa + log [ionized/unionized]

Tenidap
Henderson-Hasselbalch
Hasselbalch Plot

pKa by RP-HPLC
• Capacity Factor k’ k versus pH of mob.
phase
– Vary acetonitrile in mobile phase
– Zorbax Rx SB-C18 Column
– pH = 1 to 6.7
• pKa = inflection point from curve fitting
k’ = (lower limit + (upper limit x 10 (pH
(10 (pH-pKa)
pKa)
+ 1)
))
(pH-pKa) pKa)
))
• Lipophilicity = plateau (limits)

Tenidap apparent pKa
(37.5% acetonitrile)
un-ionized
ionized

Extrapolation to pKa

Comparison of Methods

Lipophilicity Comparison

Isotonic Solutions
Saline = 0.9% NaCl = 0.154 M
Gastric Fluid = 0.1 N HCl

Common Ion Effect
DH +
+ Cl - = DHCl = [DHCl][
DHCl] solid
D -
+ Na + = NaD = [NaD][
NaD] solid

Common Ion Effect
DH +
+ Cl - = DHCl = [DHCl][
DHCl] solid
Ksp = (DH + ) (Cl(
- ) / (DHCl(
DHCl)
D T = (DHCl(
DHCl) ) ( 1 + Ksp /Cl
- )
D T = Total Drug in Solution at each
counter ion concentration

Doxycycline pH Solubility
Bogardus, JB, et al. 1979. J. Pharm Sci 68:188-94

Doxycycline pH Solubility
Bogardus, JB, et al. 1979. J. Pharm Sci 68:188-94

Terfenadine pH Solubility
Streng, WH, et al. 1984. J Pharm Sci 73:1679-1684.

Absorption Considerations
Un-ionized Species for Passive Diffusion
Charged Species for Pump Mechanism and
ion-pairing partitioning.
Is the right species available for
absorption

Absorption Potential (AP)
• AP = log (Po/w
Fu S6.5S
V / X )
• Po/w
= Partition Coefficient
(octanol/water)
• Fu = Fraction Un-ionized at pH 6.5
• S6.5
= Total Solubility at pH 6.5 (mg/ml)
• V = Volume of lumen (225 ml)
• X = Oral Dose (mg)
J. B. Dressman, et. al., J. Pharm. Sci.73:1274-1279 (1984)

Absorption Potential
AP = log (Po/w
Fu S6.5S
V / X )
Passive Diffusion , Un-ionized Species
AP > 1 Well Absorbed
0

Absorption Potential
• AP = 1 = log (Po/w
Fu S6.5S
V / X )
• 10 = Po/w P
Fu S6.5S
V / X
• Dose in Man =X = Po/w P
Fu S6.5S
V / 10

R 1
R 2
O
N
R 3
S
O
O
N
H 2
Oxindole
Tenidap
Ilonidap
CP-100,829
R 1
Cl
F
F
R 2
H
Cl
Cl
R 3
H
H
Cl

Absorption Potential
Oxindoles
Log Po/w
Solub* AP = 1.0
Tenidap 3.95
0.041 2.7 mg
Ilonidap 4.08 0.194 6.2 mg
CP-100,829 5.60 0.043 9.0 mg
*mgA/ml (pH 6.5)

Maximum Absorbable Dose
(MAD)
MAD = ka • S6.5
• V • ( Res. Time)
ka = Absorption Rate Constant in Rats
S6.5
= Total Solubility at pH 6.5 ( mg/ml)
V = Volume of Lumen: 250 ml
Residence Time Estimate: 270 minutes

Single Pass Rat Intestinal
Perfusion

Absorption Rate Constant
k a = (1-C t /C o )Q/V
k a = absorption rate constant
C o = initial drug concentration in perfusate
C t = concentration of drug in perfusate at time t
Q = flow rate of the perfusate(0.2 ml/min)
V = volume of the lumen (πr(
2 l, 1.26 ml).

Maximum Absorbable Dose
(MAD)
Oxindoles
ka (min -1 ) MAD
Tenidap 0.027 74 mg
Ilonidap 0.037 484 mg
CP-100,829 0.045 132 mg
Oxindoles totally ionized at pH 6.5

Absorption Analysis
• Do the models support the projected
human dose
• If dose is 10X higher than MAD or AP,
then trouble with absorption

Biopharmaceutics Classification
High Solubility if the largest dose dissolves
rapidly (85% in 15 min.) in 250 ml of water in the
pH range of 1 to 8, based on solubility minimum
High Permeability if >90% of the dose is
absorbed in humans. Based on rat perfusion ka
data correlated to fraction absorbed in humans.

Biopharmaceutics Classification
Class
I
II
III
IV
Solubility
High
Low
High
Low
Permeability
High
High
Low
Low

Biopharmaceutics Classification
Class
I
II
III
IV
Rate Limiting Step
could be dissolution rate limited
dissolution rate limiting
permeability rate limiting
permeability rate limiting
Ref: Amidon, G,
Ref: Amidon, G, Pharm. Res. 12: 413
http://www.fda.gov/cder/guidance
www.fda.gov/cder/guidance/
413-420420 (1995) and

Particle Size & Shape by SEM

Effect of Particle Size on
Dissolution
Noyes-Whitney Equation
DS
vh
dc/dt
dt = k 2 ( c s - c t )
k 2
= intrinsic dissolution rate constant for this compound
D = Diffusion Coefficient
S = Total Surface Area of dissolving material
h = thickness of diffusion layer on particles
v = volume of the dissolving medium

Effect of Particle Size on
Dissolution
0.1 mm
0.5-0.7 mm
Phenacetin Particle Dissolution: 0.1 mm -> 0.6 mm
diameter

Particle Size Recommendation
K.C. Johnson and A. Swindell, Pharm. Res.13:1795-1798(1996)

Particle Size Legend

Particle Size Recommendation

Particle Size
• Particle Size Recommendation
– Considers Dissolution and Homogeneity
– Dissolution does NOT limit absorption
• Experimental
– Single Pass Intestinal Perfusion (SPIP)
– Solubility at pH 6.5
– Dose
– Particle size recommendation
• Achieving Recommended Particle Size
Ref: K. C. Johnson, A.C.Swindell, , Pharm. Res. 13: 1795-98 98 (1996)

Milling
• Reduces Particle Size
–Increases Surface Area
–Faster Dissolution
–More Surface for Reaction
–Increases Blend Homogeneity
• Imparts Amorphous Sites
–Increase Solubility
–Increase Hygroscopicity
–Decrease Chemical Stability

Initial Report
• Does this molecule possess physical
properties adequate for development
– Comparison table
– Significance of the data
– Dosage Form Development
» oral and IV dosage forms
» toxicology dosage forms
• What’s s Broken