Pharmacokinetics Exam 2 Concepts

What is a multi-compartment model?

-the body does NOT act like a single compartment

-it takes time to distribute into peripheral compartments

Multi compartment distribution (two compartment)

Point 1: end of injection (central into peripheral)

Point 2: distribution in process

Point 3: distribution equilibrium attained

Point 4: elimination process predominates

Two compartment model

method of residuals to calculate slope and intercept for each phase

One compartment model vs. multi-compartment model PK after IV infusion

multi compartment accounts for drug distribution into different tissues, leading to complex concentration-time profiles

Drug Distribution

refers to a reversible transfer of a drug between circulating blood and extra-vascular fluids and tissues of the body

-affected by: properties of drug, anatomy and physiology, disease states, other drugs

Properties of drug

-lipophilicity, size, pKa

-protein binding in plasma and tissues

anatomy and physiology

tissue composition, transporters, other barriers

other drugs

inhibitors of transporters

Drug distribution can be characterized by...

rate and extent

rate of drug distribution depends on...

-perfusion of tissue

-membrane permeability

Extent of drug distribution depends on...

-lipid solubility

-pH-pKa

-plasma protein binding

-tissue protein binding

Cyclosporine A tissue PK

-most tissues show a rapid decline in cyclosporine A concentration = fast distribution and elimination

-skin, fat, thymus exhibit initial increase in concentration before declining, possibly indicating delayed distribution or accumulation

Passive Permeability (passive diffusion)

-bidirectional (reversible)

-dependent on: properties of barrier, properties of drug, concentration gradient

Drug properties affecting membrane permeability

-size (molecular weight)

-lipophilicity

-charge (ionization state)

-being a substrate to a transporter

example of drug transporters and distribution

P-glycoprotein significantly limits distribution of ondansetron into the CNS

Perfusion-rate limitations (delivery rate limitations)

-when tissue membranes present essentially no barrier to distribution (likely for small, lipophilic molecules)

-BLOOD FLOW is a RATE LIMITING step

-well-perfused tissues take up a drug more rapidly than poorly perfused tissues

Perfusion rate in decreasing order

Kidneys (4) > Brain (0.5) > Adipose (0.025)

Permeability-rate limitations (delivery rate limitations)

-crossing the membranes is the rate-limiting step

-common for polar drugs

-physiochemical properties of the drug can significantly influence the permeability (pKa, LogP, LogD)

Vd vs. Physiological Volumes

breaks down total body water into three main compartments

1. plasma

2. interstitial fluid

3. intracellular fluid

-fluid intake and output (via kidneys, lungs, feces, sweat, and skin)

Volume of Distribution (Vd)

describes relationship between amount of drug in the body and the concentration in the blood (plasma)

-proportionally constant

-has no direct physiological meaning

-Vc, Vss, Varea

Apparent Volume of Distribution

-numerical value may be similar to a physiological space in the body

-apparent volume DOES NOT necessarily correspond to a physiological volume

Why are we interested in Vd?

it can be used for calculating the dose

Dose = C x Vd

Protein Binding

- Drug + Protein = Drug Protein Comlex

-fraction unbound

-fraction bound

-most methods measure Ctotal

-total amount of drug in the body is independent of measurement method

acidic drugs commonly bind to ablumin

basic drugs commonly bind to alpha-1 acid glycoprotein

Between Subject variability in fu and Vd (propanolol example)

different health conditions can affect how much of a drug is free in the blood and how widely it spreads in the body

-as alpha 1-acid glycoprotein levels increase, fraction unbound decreases (varies across disease states)

Extent of Distribution

-Vd increases when fu,p is increased

-Vd decreases when Fu,t is increased

-fu,t is the average value across all tissues into which drug distributes

-Kp can vary considerably among tissues

Different values of Vd

Vc (V1) - volume of the central compartment

Vextrap - extrapolated volume

Varea - volume at the terminal phase

Vss - volume at steady-state

value dependent on quality of the observed data

Multicompartment distribution

Varea > Vss > Vc

Two compartment model graph

use method of residuals to calculate C1

-use of back-extrapolation of the terminal phase slope will overestimate the volume of distribution

Varea, Vbeta, Vz

-volume calculated at the terminal elimination phase

-uses area under the concentration-time curve

-influenced by changes in clearance

-this is a method for calculation and does not mean that clearance is dependent on the volume of distribution

Volume of distribution at steady state, Vss

-not affected by clearance

-calculation using exponential parameters

Varea after extravascular administration

the dose that gains access to the systemic circulation might be less than 100%

Vd is time dependent during the...

distribution phase

How to identify Non-Linear PK

-systematic trends observed in dose normalized PK profile and NCA parameters with non-linear PK

-greater than dose-proportional increases in AUC with dose suggests saturable clearance mechanism

General Expectations of Linear Antibody Pharmacokinetics

-biexponential PK profile typically observed

-long terminal half-life (2-3 weeks)

-low volume of distribution

Characteristics of mAbs with 'typical' PK

-no target mediated drug disposition

-typically bind to soluble targets

General expectations of non-linear antibody PK

-greater than proportional increases in AUC

-dose dependent changes in CL, Vd, t1/2

Characteristics of mmAbs with non-linear PK

-often due to target-mediated drug disposition

-typical targets are either

membrane bound - HER2, EGFT, CD20

soluble and form immune complexes - IgE

IgG Elimination is Concentratoin-Dependent

-inverse relationship between serum IgG concentrations and half life

-Brambell proposed that this observation was due to the presence of a salvage receptor for IgG

Neonatal Fc Receptor - FcRn

-protection receptor for IgG

-functions as a 65 kDa heterodimer

1. 15 kDa light chain - Beta2 microglobulin

2. 50 kDa heavy chain - MHC1-like protein

-expressed throughout the body

-endogenous roles

1. IgG/albumin homeostasis

2. maternal transfer of IgG to fetus/neonate

How does FcRn protect IgG from catabolism?

1. proteins are internalized via fluid-phase pinocytosis

2. endosomal acidification causes protonation of key histidines in IgG and FcRn

3. protonation leads to favorable IgG-FcRn binding

4. IgG-FcRn complexes are recycled to the cell surface

5. following exposure to extracellular pH, IgG-FcRn complexes dissociate

FcRn function?

protection of IgG from catabolism

Oral Route of mAb therapeutics

bioavailability - negligible

tmax - NA

Barriers - low gastric pH, GI tract enzymes, GI eptihelium

Intravenous route of administration mAb therapeutics

Bioavailability - 100%

tmax - immediately

barriers - NA

Subcutaneous and Intramuscular route of administration mAb therapeutics

bioavailability - 52-80%

tmax - 6-8 days

barriers - lymphatics, immune cells

SC administration is very attractive for pharmaceutical development

-absorption is largely via lymphatics

-reasonable bioavailability

-less pain compared to IM injections

-more convenient to patients compared to IV

-significant investment into absorption enhancing strategies

General expectations - PK following SC dosing

-relatively slow absorption

-reasonable bioavailability

-often only see monoexponential PK (first phase may be masked by absorption)

What happens to the bioavailability of mAbs at high dose levels?

it decreases

What happens to bioavailability after genetic knockout of FcRn?

reduces bioavailability of mAbs to 30%

Diffusion Pathway for IgG Distribution

Efficiency - negligible

Barriers - cell membrane

Bulk fluid flow pathway for IgG Distribution

Efficiency - tissue dependent

Barriers - endothelial pores, interstitial pressure

Pinocytosis Pathway for IgG distribution

Efficiency - relatively low

Barriers - endocytic rate

Receptor Mediated Uptake Pathway for IgG Distribution

Efficiency - target dependent

Barriers- target expression, target accessibility, endocytic rate, binding affinity

Tissue concentrations is LESS THAN plasma concentrations

-slow extravasation into tissues

-relatively rapid drainage via lymphatics

Small Vss from NCA analysis

-typically close to plasma volume

-True Vss measurement requires tissue concentrations of mAbs

IgG elimination

-intracellular catabolism

-target mediated elimination (specific)

-receptor-mediated elimination

-receptor mediated protection: FcRn

-immunogenicity

Intracellular catabolism

-following fluid phase endocytosis (non-specific)

-limited by interactions with FcRn

Target mediated elimination (specific)

-cell surface receptor: internalization

Soluble target: formation of large complexes - phagocytosis

Receptor mediated elimination

-may trigger endocytosis and catabolism

Receptor mediated protection - FcRn

level of plasma concentration of IgG

Immunogenicity elimination

formation of anti-drug antibodies (ADA) resulting in accelerated clearance or loss of activity

What about renal clearance?

-generally accepted molecular weight cutoff for glomerular filtration - 60 kDa

- in healthy individuals, <0.01% of IgG in serum is expected to pass through the glomerulus

Key features of elimination following pinocytosis

-non specific

-can occur in any cell

-results in catabolism to component amino acids

-efficiency is blunted by FcRn recycling

-at typical mAb doses, linear process

Key features of target mediated elimination

-highly specific

-saturation level and rate depends on: target expression/accessibility, target turnover

-occurs following receptor binding/internalization

-typically a non-linear process (manifests as dose dependent changes in CL/Vss)

At low doses...

any target mediated processes will dominate the profile

-dose dependent decreases in CL until saturation

at extremely high doses, FcRn will be...

saturated

-dose dependent increases in CL

Soluble Target types

generally present in circulation (VEGF)

Membrane, non-internalizable

target is expressed on the cell surface but does not endocytose

Membrane, internalizable

target is expressed on the cell surface but endocytoses

Membrane, sheddable

target is expressed on the cell surface but may be release into the extracellular space

Do target properties influence PK of mAbs?

YESSSS

Immunogenicity refers to...

the formation of antibodies against the therapeutic molecule

-referred as anti drug antibodies

-recognition of the therapeutic as foreign by the immune system

Different types of ADA

clearing - adverse effects on PK

neutralizing - inhibits binding to target/efficacy

non clearing/non neutralizing - no impact

Factors associated with immunogenicity

1. duration of therapy

2. dose

3. route of administration

4. frequency/onset of ADA cannot be predicted

duration of therapy

-incidence typically increases with treatment time

Dose

ADA more frequently detected at lower does of mAb

Route of administration

ADA often found more frequently with SC/IM dosing

If profiles do not overlay...

it is non linear PK

for mabs, non linear PK is often attributable to...

target-mediated disposition

Mechanism of Target Mediated Drug Disposition (TMDD)

high affinity binding of a large fraction of the dose to its pharmacologic target will significantly impact PK (hypothesis made based on the increasing number of drugs in development with high affinity for targets)

General Expectations of TMDD

-NCA derived Vss and Cl will decrease with increasing dose to a limiting value

-greater than dose proportional changes in AUC at low doses

Key features of TMDD for mabs

non linear PK

dose dependent decreases in clearance

-typically observed for mabs against cell surface targets

-TMDD is saturable in nature

PK studies at a range of doses are REQUIRED to...

characterize TMDD

TMDD model is able to well characterize...

1. plasma TRX1 PK

2. free and total CD4 concentrations

model-estimated affinity DOES NOT agree wtih...

in vitro measurements

TMDD models are useful to characterize...

blood PK data

Physiologically-based PK models allow

1. integration of mechanistic determinants of mab PK

2. description of both blood and tissue PK

3. relatively straightforward scaling bw species

PBPK challenges include

1. difficult model validation

2. complex system of equations

gold standard for prediction of PK

more useful for prediction than fitting

Antibody Drug Conjugates

combine selectivity of mabs with the potency of chemo drugs

-careful selection of target antigen is critical to avoid severe toxicities

ADC PK measurements require assays for several analytes

1. conjugated mab

2. free/total mab

3. free/total drug

drug to antibody ratio affects PK/PD

1. increased DAR leads to faster clearance

2. some degree of deconjugation can be expected with time

Bispecific antibodies

affinity for

1. several molecular formats can be produced

2. generally, include fragments from two dif mabs

bsAb PK

-eliminated very quickly

-have to consider target kinetics/cycling

Biosimilars

biological product that his HIGHLY similar to and has NO clinically meaningful differences from an existing FDA approved reference product

If the drug is water soluble and not highly bound to plasma proteins, it can be...

directly eliminated

most drugs are too ______ to be directly eliminated

lipophilic

Phase 1 Metabolism

CYP enzymes; many phase I metabolites are active

Phase II Metabolism

phase II metabolites are usually water soluble and inactive

Drug elimination can only be measured in the clinic by analyzing...

changes in drug concentration over time in blood, urine, breath samples, etc.

volume of blood completely cleared of drug per unit time

considers entire body to be a single drug eliminating system made up of one or more elimination processes

-does not identify mechanism of elimination

What is the most useful measure of drug elimination and why?

Clearance; because it remains constant (it is NOT concentration dependent)