Lipinski rule detailed note
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Lipinski rule five criteria molecular weight logP hydrogen bond donors acceptors ADME drug discovery exceptions beyond rule of five
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Lipinski's Rule of Five (Ro5) - Detailed Note
1. Background and Origin
Lipinski's Rule of Five (also called Pfizer's Rule of Five or Ro5) was formulated by Christopher A. Lipinski, a chemist at Pfizer, and published in 1997 (later formally cited as Lipinski et al., Adv. Drug Deliv. Rev., 2001, 46:3-26). It arose from Lipinski's analysis of physicochemical properties of compounds in the World Drug Index that had reached at least Phase II clinical trials.
The core observation: most orally administered drugs are relatively small and moderately lipophilic molecules. Lipinski codified this observation into a simple filter for predicting whether a compound is likely to be orally bioavailable in humans.
2. The Four Core Criteria
The rule states that an orally active drug will generally have no more than ONE violation of the following:
| Parameter | Threshold | Rationale |
|---|---|---|
| Molecular Weight (MW) | ≤ 500 Da | Larger molecules diffuse more slowly across membranes; passive diffusion rate decreases as size increases |
| LogP (lipophilicity) | ≤ 5 (cLogP ≤ 5) | Too hydrophilic = poor membrane penetration; too lipophilic = poor aqueous solubility |
| Hydrogen Bond Donors (HBD) | ≤ 5 | NH and OH groups; excess donors impede membrane crossing |
| Hydrogen Bond Acceptors (HBA) | ≤ 10 | N and O atoms; excess acceptors impede membrane crossing |
Why "Rule of Five"? All the numbers (500, 5, 5, 10) are multiples of 5 - hence the name.
Note: Some sources also include a 5th criterion (added later): rotatable bonds ≤ 10, which relates to molecular flexibility and oral absorption.
3. Molecular Basis - Why These Parameters Matter
Molecular Weight
- Passive diffusion through the lipid bilayer of the GI epithelium is size-dependent
- Molecules >500 Da are generally too large to passively diffuse through intestinal membranes
- MW relates directly to the number of atoms and the "footprint" the molecule presents to a membrane
LogP (Octanol-Water Partition Coefficient)
- LogP measures lipophilicity - the tendency of a compound to partition between octanol (fat-like) and water
- An ideal drug needs balanced hydrophilicity and lipophilicity:
- Too low LogP (very polar): poor membrane permeability
- Too high LogP (very nonpolar): poor aqueous solubility, poor absorption from the GI tract, risk of metabolic instability
- cLogP = calculated LogP; aLogP = atomic contribution-based LogP
Hydrogen Bond Donors (HBD)
- NH and OH groups that can donate H-bonds to water molecules
- Excess HBDs make the compound too "sticky" to water, preventing it from partitioning into the lipid membrane
- Each additional HBD costs approximately 1 log unit in permeability
Hydrogen Bond Acceptors (HBA)
- Nitrogen and oxygen atoms that can accept H-bonds
- Similar mechanism: too many HBAs increase polarity and reduce membrane permeability
- Note: N and O atoms count even when not directly involved in H-bonding
4. ADME Relevance
The Ro5 specifically addresses the "A" in ADME - Absorption. It is fundamentally a filter for oral bioavailability via passive transcellular diffusion across the GI epithelium.
As confirmed in Goodman & Gilman's Pharmacological Basis of Therapeutics: "Decades of experience have led to a number of rules of thumb for what makes a compound 'drug-like', such as the 'rule of five' (Lipinski et al., 2001). These may be useful guides during drug discovery projects."
The Ro5 does NOT predict:
- Pharmacological activity or potency
- Hepatic clearance or half-life
- Safety or toxicity
- Drug-drug interactions
- CNS penetration (blood-brain barrier crossing)
- Distribution or metabolism
5. Practical Application in Drug Discovery
In drug discovery, lead optimization involves progressively modifying a hit compound to improve binding affinity and selectivity. This process often increases MW and LogP, which can push a compound outside Ro5 compliance. The Ro5 serves as a guardrail throughout this process.
Ro5-compliant candidates tend to show:
- Lower attrition rates in clinical trials
- Higher probability of reaching the market
- Better predicted oral absorption from GI tract
6. Exceptions and Limitations
Known Exceptions (Drugs that violate Ro5 but are orally bioavailable)
| Drug | MW (Da) | LogP | Violation | Use |
|---|---|---|---|---|
| Venetoclax | ~868 | >6 | MW + LogP | Oncology (BCL-2 inhibitor) |
| Montelukast | ~586 | >7 | MW + LogP | Asthma |
| Cyclosporin A | ~1203 | ~3.4 | MW + HBA | Immunosuppressant |
| Azithromycin | ~749 | ~4 | MW | Antibiotic |
These exceptions are often explained by:
- Active transporters (P-glycoprotein, OATP, etc.) facilitating intestinal uptake
- Transcellular active transport rather than passive diffusion
- Special formulations that improve bioavailability
Fundamental Criticism
- The rule assumes passive transcellular diffusion as the sole absorption mechanism, ignoring:
- Carrier-mediated transport
- Paracellular transport
- Efflux pump effects (P-gp, BCRP)
- About 50% of orally administered new chemical entities do not actually comply with the Ro5 yet are still orally bioavailable (O'Hagan et al.)
- Natural products (macrolides, peptides, cyclic compounds) frequently break Ro5 filters
7. Extensions and Variants
Veber's Rule (2002)
Focuses on topological polar surface area (TPSA) and rotatable bonds rather than MW:
- TPSA ≤ 140 Ų (or ≤ 12 H-bond donors + acceptors)
- Rotatable bonds ≤ 10
- Better discriminator for large compound datasets
Ghose Filter
- LogP: -0.4 to +5.6
- MW: 160-480 Da
- Molar refractivity: 40-130
- Number of atoms: 20-70
Rule of Three (Ro3) - Lead-like Compounds
For defining lead-like (not drug-like) compounds in screening libraries:
- MW ≤ 300 Da
- LogP ≤ 3
- HBD ≤ 3
- HBA ≤ 3
- Rotatable bonds ≤ 3
- TPSA ≤ 60 Ų
This accounts for the fact that lead optimization always increases MW and LogP, so starting from a smaller, simpler compound gives more "room to grow."
Extended Rule of Five (eRo5) / Beyond Rule of Five (bRo5)
For newer drug classes (macrocycles, PROTACs, biologics-derived):
- MW ≤ 1000 Da
- HBD ≤ 6
- HBA ≤ 15
- cLogP: -2 to +10
- Usually macrocyclic structures
These target difficult protein-protein interaction (PPI) interfaces that are "undruggable" by classical small molecules.
8. Summary Table
| Rule | MW | LogP | HBD | HBA | Extra |
|---|---|---|---|---|---|
| Lipinski Ro5 | ≤500 | ≤5 | ≤5 | ≤10 | - |
| Ro3 (Lead-like) | ≤300 | ≤3 | ≤3 | ≤3 | Rotatable bonds ≤3 |
| Veber's Rule | - | - | - | - | TPSA ≤140, Rotatable bonds ≤10 |
| bRo5 | ≤1000 | -2 to 10 | ≤6 | ≤15 | Usually macrocycles |
9. Key Points to Remember
- Ro5 = filter for oral bioavailability via passive diffusion, not a guarantee of drug efficacy
- One violation is acceptable - compounds with 2+ violations are considered poor drug candidates
- All cutoffs are multiples of 5 (500, 5, 5, 10)
- Applies only to small molecule drugs, NOT to biologics, peptides, or prodrugs designed for active transport
- Natural products, macrolides, and cyclic peptides often violate Ro5 yet remain clinically useful
- The rule remains the most widely used in silico filter in medicinal chemistry and high-throughput screening
- Lipinski himself later published a "relaxed" version acknowledging the evolution of drug space beyond Ro5
Source: Goodman & Gilman's Pharmacological Basis of Therapeutics (Medicinal Chemistry section); Lipinski et al., Adv. Drug Deliv. Rev. 2001, 46:3-26; Wikipedia - Lipinski's rule of five; MedChemica - Beyond Rule of Five
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