1. What is Absorption?
In drug discovery, absorption is the process by which a drug transitions from its site of administration (most commonly the gastrointestinal lumen after oral dosing) into the systemic circulation. To achieve this, a dissolved drug molecule must cross the semi-permeable epithelial cell membrane of the intestinal wall. This movement occurs primarily via passive transcellular diffusion (dissolving through the lipid bilayer) or paracellular transport (passing between tight junctions), though it can also involve carrier-mediated active transport (influx pumps) or encounter efflux mechanisms (active rejection back into the lumen).
2. Assays Used to Test Absorption
In early drug discovery, absorption is evaluated using a combination of in silico, in vitro, and in vivo profiling methods:
Kinetic & Thermodynamic Solubility Assays: Quantify how much drug dissolves in phosphate-buffered saline (PBS) and biorelevant media like FaSSIF and FeSSIF (fasted/fed state simulated intestinal fluids).
PAMPA (Parallel Artificial Membrane Permeability Assay): Utilizes a lipid-infused synthetic membrane to isolate and measure pure passive transcellular permeability without cellular variables.
Caco-2 / MDCK-MDR1 Cell Monolayers: Cultured biological cell lines that mimic the intestinal barrier. By measuring transport from the apical-to-basolateral and basolateral-to-apical directions, scientists calculate the Efflux Ratio (ER=(P(B-A)/P(A-B)). A ER> 2.0 typically flags active efflux by transporters like P-glycoprotein (P-gp) or BCRP.
In Vivo Rodent PK: The ultimate test, measuring absolute oral bioavailability by comparing plasma concentration-time curves of oral (PO) vs. intravenous (IV) dosing.
3. Typical Issues in Drug Discovery
Absorption failures generally stem from the physical chemistry of the lead matter, categorized by the Biopharmaceutics Classification System (BCS):
Poor Dissolution / Solubility (BCS Class II): Highly lipophilic, “brick-dust” compounds with high crystal lattice energy that refuse to dissolve in aqueous GI fluids.
Poor Permeability (BCS Class III): Hydrophilic, highly polar, or excessively large molecular weight compounds ($MW > 500\text{ Da}$) that cannot passively partition into the lipophilic lipid bilayer.
Active Efflux (BCS Class IV/Generic): Compounds that dissolve and enter the membrane but are immediately recognized and pumped back out by apical ATP-binding cassette (ABC) transporters (e.g., P-gp), preventing systemic exposure.
4. How to Solve These Issues
Medicinal chemists and formulation scientists deploy targeted strategies based on the specific bottleneck identified:
| Bottleneck | Medicinal Chemistry Strategies (Structural) | Formulation & Form Strategies |
| Low Solubility | • Introduce asymmetry or twist planarity to disrupt crystal packing. • Add basic or acidic ionizable groups. • Lower LogP by replacing carbons with heteroatoms (N, O). | • Salt Screening: Prep crystalline salts (e.g., HCl, mesylate, sodium) to boost dissolution rate. • Amorphous Dispersions: Disrupt crystal structure using polymer matrices. • Cyclodextrin inclusion complexes. |
| Low Permeability | • Reduce Topological Polar Surface Area (TPSA < 120A). • Remove or cap solvent-accessible Hydrogen Bond Donors (HBD). • Prodrug approach: Mask polar carboxylic acids as lipophilic esters. | • Permeation Enhancers: Use surfactants or fatty acids to transiently disrupt tight junctions (primarily for specialized delivery). |
| High Efflux | • Remove specific strong HBDs or reduce basicity of amines (e.g., swap a secondary amine for morpholine or introduce fluorine). | • Co-administer excipients that act as mild P-gp inhibitors (e.g., TPGS, PEG-400) during early in vivo animal screening. |
Brief Summary
Optimizing drug absorption requires a precise balance of molecular properties. Highly insoluble compounds require structural disruption of their crystal lattice or the use of enabling formulations (salts, amorphous solid dispersions) to drive dissolution. Conversely, poorly permeable compounds demand the reduction of polar surface area and hydrogen bond donors to allow passive membrane diffusion. By deploying parallel in vitro screens (Solubility, PAMPA, Caco-2) early in the design loop, discovery teams can iteratively engineer lead molecules that possess both target potency and the necessary biophysical properties for robust oral bioavailability.