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Entropy-Reinforced Planning with Large Language Models for Drug Discovery
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Proposes ERP, an MCTS-based planner with an e-step forward entropy bonus and Top-PK expansion that guides a GPT-2 SMILES Transformer decoder to generate drug-like molecules with better multi-property rewards, and also tests it on APPS code generation.
Problem
LLM decoding (beam search, sampling) for molecule generation yields many invalid molecules and suboptimal candidates for multi-property drug discovery, and prior MCTS-guided decoding (PG-TD) over-exploits high-probability tokens and wastes search on unpromising molecules.
Approach
Augments the P-UCT selection bonus with the Transformer's next-token probability multiplied by the average entropy of the policy over the next e steps, so the planner explores uncertain subtrees; expansion uses Top-PK filtering and evaluation uses beam search completions scored by a multi-critic normalized reward.
Key results
- On 3CLPro, ERP achieves best normalized reward 3.24 (+13.71% over sampling) versus PG-TD 3.20 (+12.50%).
- On RTCB, ERP achieves avg valid normalized reward 2.61 (+5.61% over sampling), with PG-TD at 2.53 (+2.26%).
- On APPS introductory with GPT-2, ERP improves 1@20 pass rate to 25.204% versus PG-TD 23.540%.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| use ERP instead of PG-TD | Pretrained GPT-2 molecule generation · 3CLPro (7BQY) | Average valid normalized reward: 2.59 -> 2.62 | Helped |
| use ERP instead of PG-TD | Pretrained GPT-2 molecule generation · RTCB (4DWQ) | Average valid normalized reward: 2.53 -> 2.61 | Helped |
| use ERP instead of PG-TD | GPT-2 code generation · APPS Intro pass@15 | Pass rate (%): 27.873 -> 28.421 | Helped |
| remove Top-P-K filtering during expansion (ablation) | ERP with e=6 · 3CLPro (7BQY) | Average normalized reward: 2.60 -> 2.46 | Hurt |
| set forward entropy steps to 0 (ablation) | ERP with Top-P-K filtering · 3CLPro (7BQY) | Average normalized reward: 2.60 -> 2.57 | Hurt |
| set forward entropy steps to 1 (ablation) | ERP with Top-P-K filtering · 3CLPro (7BQY) | Average normalized reward: 2.60 -> 2.57 | Hurt |
| increase forward entropy steps from e=2 to e=3 (ablation) | ERP · APPS Intro pass@20 | Pass rate (%): 29.711 -> 29.991 | Helped |
| increase forward entropy steps from e=2 to e=4 (ablation) | ERP · APPS Intro pass@20 | Pass rate (%): 29.711 -> 29.990 | Helped |
Surprising findings
- PG-TD's performance dropped when rollouts increased from 256 to 512, while ERP improved monotonically.
- Biased-model sampling performed significantly worse than pure MCTS UCT, indicating biased priors steer generation into low-reward regions.
Limitations
- Docking scores come from a surrogate BERT-like model rather than physical docking.
- Code generation experiments used reduced rollouts (64) due to limited compute, and differences are small (often within a few tenths of a percent).
- Reported improvements over sampling are modest on some metrics; ERP does not win every metric (e.g., unique valid molecules on 3CLPro vs UCT).
Experimental setup
Datasets: ZINC ZINC20 Cancer and COVID docking dataset (3CLPro, RTCB) APPS CodeContests
Benchmarks: 3CLPro (PDB 7BQY) molecule generation RTCB (PDB 4DWQ) molecule generation APPS Intro/Inter/Comp CodeContests
Metrics: best normalized reward average valid normalized reward average top 10% normalized reward unique valid molecules docking score druglikeness synthesizability solubility pass rate strict accuracy
Baselines: Beam Search Sampling UCT PG-TD
Builds on: PG-TD UCT P-UCT (value-guided MCTS) beam search
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- Searching for High-Value Molecules Using Reinforcement Learning and Transformers (ICLR 2024 · similarity 0.85)
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