**4.1 BBB permeation data**

#### **4.1.1 Bound and unbound drug concepts**

The drug is available in blood in the free (unbound) and bounded (protein bounded, erythrocyte bounded, tissue bounded) forms. The unbound drug molecules equilibrate across the BBB and brain. The spaces that these equilibria occur are: blood, interstitial fluid, intercellular and intracellular fluids. Figure 5 shows these equilibria schematically. The speed of the equilibria to reach the steady state define the rate of drug distribution within brain, and the slowest one would be the rate limiting step. For poor CNS penetrantes, the BBB permeation or the diffusion of drug molecules within the brain tissue is the rate limiting step. Total brain concentration which allow us just to rank drug candidates according to their CNS total levels and general CNS penetrability can be measured using most of the *in vivo* methods, while there is just a few methods which are able to provide free fractions directly.

Blood Brain Barrier Permeation 13

Brain uptake index (BUI%) is one of the earliest indicators of BBB permeability of

% 100

where E denotes the first pass extraction and the Eref referred to freely diffusible internal standard. This indicator provides information about the total concentration of the drug in

The logBB which describes the ratio between brain and blood (or plasma) concentrations and provide a measure of the extent of drug permeation is calculated using (Kerns & Di,

> log *tot.brain p*

The only information provided by Kp is passive lipid partitioning of the drug which is affected by metabolism, relative binding affinity to proteins and lipid content of brain and blood or plasma and it is not a net measure of BBB permeability (Abbott, 2004; Mehdipour & Hamidi, 2009). It is highly time dependent and in order to get an overall estimation, usually

Another approach based on unbound drug fraction, for quantifying the extent of brain

*AUC*

Kp,uu affected by both passive diffusion and active influx/efflux and can give information about the permeation mechanism, beyond these, it is not affected by plasma protein and brain tissue binding which interfere in logBB values (Mehdipour & Hamidi, 2009). For drugs delivered by passive diffusion, this index will be close to unity while for efflux and influx substrates it will be less than and more than unity respectively (Hammarlund- Udenaes et

To assess the brain drug permeability rate, the unidirectional influx constant from blood to the brain (Kin) and the product of the BBB permeability surface area (PS) which is a measure of the unidirectional clearance from blood to brain have been developed. Both parameters expressed as ml/min/g of brain (Rooy et al., 2010). PS is able to reflect the BBB permeation step more accurately (Abbott, 2004) and is valuable parameter for follow up permeation ability of drug candidates in the pharmaceutical industry and although in pathologic conditions. PS gives an estimation of unbound drug in brain but it is affected by the possible association of

According to the measurement method Kin and PS can be calculated from Crone-Renkin

1

*<sup>F</sup> KF e in* 

*PS*

the drug with active influx or efflux transporters (Hammarlund- Udenaes et al., 2008).

*p,uu*

*K* 

*AUC BB or K* 

the brain at early time point after administration (Lanevskij et al., 2010).

*<sup>E</sup> BUI*

*ref*

*tot. blood*

u, blood

*<sup>E</sup>* (1)

*AUC*  (2)

*AUC* (3)

(4)

compounds and is calculated by:

is measured under steady-state conditions.

penetration is recommended, which is calculated by:

u, brain

2008):

al., 2008).

equation:

Fig. 5. Different equilibria in brain.
