Absorption
Currently, the model accommodates four routes of administration - oral, intramuscular (IM), intravenous (IV) and subcutaneous (SC) only. The
options for absorption inputs follow based on the selected route of administration.
⋇ Oral administration
On selecting oral route of administration, two options for absorption appear. The first one is absorption type and the second to enter the
respective value.
The most common in vitro experiments to assess the drug's apparent permeability (P app) through the small intestine across the
apical-basolateral layer is using cell monolayers such as Caco-2 or MDCK cells. The values are generally reported as cm.s⁻¹. If this data is not
readily available, polar surface area (PSA) and hydrogen bond donors (HBD) can be used.
Polar surface area (PSA) is the sum of the surface area of all the polar atoms consisting mainly oxygen and nitrogen including any attached
hydrogen atoms in a drug. The value entered should be in
sq. angstroms (Ų).
Hydrogen atoms connected to the donor sites are considered as hydrogen bond donors (HBD). The input value in the model takes the
number of hydrogen bond donors.
The effective permeability (Peff, in cm.s⁻¹) of the drug is the in vivo permeability in humans. This Peff is
computed using IVIVE obtained from various literature sources as follows:
The rate of absorption (Ka, in h⁻¹) in humans is computed using Peff as follows3:
The user is required to select the type of absorption input that is being provided. The corresponding absorption value should be entered for the selected absorption type (in the appropriate unitsi.e. either in ⨯10⁻⁶ cm.s⁻¹ or Ų or h⁻¹). If there is no apparent permeability available for the drug, an option to enter the Kₐ value is also provided. This scalar would multiply the absorption rate (Kₐ) by the provided input.
There is also an option to adjust the absorption value using the optional box under the 'Optional tab' to input any scalar ('Absorption scalar').
Please note: This absorption value would remain constant for the entire simulation. Currently, the model does not have any transporter data
or induction/inhibition included.
Note: If the absorption value is 15 ⨯ 10⁻⁶ cm.s⁻¹, please enter only 15. The model would then multiply this value by ⨯10⁻⁶.
⋇ Intramuscular
Upon selection of intramuscular (IM) administration, an input box to include IM release rate is provided. By default, the value is 0.0 and the
units are in h⁻¹.
The model currently assumes a first-order release rate from the IM depot (a standard depot volume of 4 mL is assumed for humans and 1 mL for rats and mice) which releases the drug into the surrounding muscle that subsequently distributes across the body as described in Rajoli et al.4
⋇ Subcutaneous
The model follows a similar pattern as the intramuscular administration with a first-order release from the subcutaneous compartment, but the drug releasing into the surrounding adipose compartment.
⋇ Intravenous
Selection of an intravenous (IV) route does not require further absorption inputs as the model considers the provided dose as IV bolus.