Binding potential
Binding potential (BP) is the ratio of
Bmax
(receptor density) to
KD
(radioligand equilibrium dissociation constant), as defined by Mintun et al. (1984).
BP = Bmax / KD = receptor density x affinity.
In vitro at tracer doses, BP equals the ratio of specifically bound ligand to its free concentration.
In vivo not all of receptors are available for the radioligand
to bind because of occupance by endogenous ligands. Therefore, in in vivo
imaging the term Bavail is often used instead of Bmax.
Assuming that radioligand is not actively transported into tissue, so that at
equlibrium the free concentration in plasma (CFP) equals the
free concentration in tissue (CFT),
BPF = Bavail / KD = CS / CFT
= CS / CFP
In terms of volumes of distribution,
BPF = (VT - VND) / fP
, where VT and VND could be estimated
from region-of-interest and reference region, respectively, for example with
Logan plot. Calculation of BPF requires plasma sampling and
measurement of plasma protein binding (plasma free fraction
fP).
Usually, the determination of fP is omitted because it is
prone to errors and would merely add more variability to results. Then,
the specific binding in tissue is compared to not free but total plasma
concentration:
BPP = VT - VND
= fP·Bavail / KD
If specific uptake is not compared to plasma concentrations but to
non-displaceable
tissue uptake, we can determine BPND:
BPND = (VT - VND) / VND
= VT / VND - 1
, where the term VT / VND is often referred to as
distribution volume ratio (DVR).
DVR can be estimated without plasma sampling
using for example (simplified) reference tissue compartment model or Logan plot
with reference tissue input.
However, BPND could also be calculated from distribution volumes
that are estimated using plasma sampling.
See also:
- Calculation of reference tissue input compartment models for regional data
- Calculation of BPND images
