BOAT: Navigating the Sea of In Silico Predictors for Antibody Design via Multi-Objective Bayesian Optimization
1. BOAT is a plug-and-play multi-objective Bayesian optimization (BO) framework that jointly optimizes multiple antibody properties predicted by arbitrary in silico “oracles,” aiming to replace inefficient sequential filtering pipelines with Pareto-aware design.
2. The key engineering idea: uncertainty-aware surrogate modeling (Gaussian processes) proposes which sequences to score next, while a genetic algorithm (GA) is used to optimize acquisition functions directly in discrete sequence space (avoiding invalid continuous edits and awkward projections).
3. BOAT targets realistic lead-optimization settings where objectives can conflict (e.g., affinity vs. developability vs. immunogenicity risk proxies). It supports full-sequence or region-restricted optimization (e.g., specific CDRs), plus practical constraints such as restricting mutable positions, allowed amino-acid dictionaries, and liability filtering (e.g., glycosylation motifs).
4. Method details: sequences are embedded (one-hot, BLOSUM-derived, bag-of-5-grams, or AbLang-2 embeddings), then modeled with a GP using a Tanimoto kernel to better handle high-dimensional sparse-like representations. Multi-objective acquisition uses EHVI (and NEHVI for noisy settings), implemented via BoTorch.
5. Cross-reactive VHH case study: BOAT optimizes CDR1/2/3 (up to 5 mutations per CDR) to improve binding to two related antigens, optionally adding humanness (OASis) and PLM likelihood (ESM-2) as additional objectives. Mutation choices are constrained to a curated per-position amino-acid dictionary grounded in available experimental single-point data.
6. Benchmarking against GA baselines (sum-of-objectives GA and NSGA-II): across 2–4 objectives and multiple CDRs, BOAT variants reach higher hypervolume earlier and end with better hypervolume under the same oracle-call budget (1000). NSGA-II degrades notably as objective count increases, consistent with many-objective optimization issues.
7. When exhaustive enumeration is feasible (smaller constrained spaces), BOAT recovers Pareto fronts close to the “ground-truth” oracle-induced Pareto frontier, including in very large enumerated CDR3 spaces (tens of millions of sequences), highlighting sample-efficient Pareto exploration rather than brute-force scoring.
8. Diversity matters for wet-lab follow-up: batch BO acquisition (qEHVI/qNEHVI) tends to produce higher Shannon-entropy sequence sets while maintaining strong hypervolume, whereas sequential EHVI can be more exploitative (competitive hypervolume but lower diversity). Larger batch sizes increase diversity, with some early hypervolume trade-offs.
9. Practical limits and regimes: (i) NEHVI can become dramatically slower as objectives increase (e.g., 3 objectives taking minutes per BO step vs seconds for 2), (ii) complex structure-based oracles (Boltz-2 ipTM) can break surrogate fidelity with simple encodings—here, semi-random GA search can be competitive, motivating richer structure-aware surrogates/kernels.
10. Comparison to generative multi-objective methods (LaMBO-2) on the 4-4-20 scFv affinity/expression dataset: using the same discriminative head as BOAT’s oracle, BOAT generally achieves higher hypervolume over generated sequences. However, BOAT can exploit predictor artifacts and go out-of-distribution; adding an ESM-2 likelihood objective acts as a “naturalness” regularizer, underscoring that oracle quality and priors critically shape in silico Pareto fronts.
💻Code:
github.com/AstraZeneca/boat
📜Paper:
arxiv.org/abs/2604.13980
#BayesianOptimization #MultiObjectiveOptimization #AntibodyDesign #ProteinEngineering #MachineLearning #ComputationalBiology #DrugDiscovery #ActiveLearning #GaussianProcesses #ParetoOptimization