Pharmaceutical lead optimization traditionally integrates potency, selectivity, exposure, safety, physicochemical behavior, and synthetic feasibility, yet the environmental implications of a candidate are often considered only after substantial structural and process commitments have been made. This separation is scientifically problematic because molecular features selected during discovery can influence environmental transformation, persistence, mobility, bioaccumulation, ecotoxicological activity, manufacturing burden, and the properties of transformation products. This article develops Sustainability-Aware Lead Optimization as an original conceptual contribution for treating these consequences as a structured part of candidate design rather than as an end-stage annotation. The approach centers on a proposed Candidate Environmental Profile that preserves distinct evidence streams for environmental fate, ecological exposure and effects, therapeutic quality, synthesis, lifecycle burden, uncertainty, and decision provenance. The profile is intended to support multiobjective comparison, targeted evidence generation, and explicit governance gates without collapsing heterogeneous dimensions into a universal sustainability score. The synthesis shows why environmental performance cannot be inferred from a single descriptor, biodegradation result, hazard endpoint, or computational prediction, and why apparent environmental improvement may be offset by reduced therapeutic performance, increased dose, difficult synthesis, hazardous processing, or burden shifting across the lifecycle. The contribution remains conceptual: it does not establish predictive validity, regulatory acceptability, or operational readiness. Its value lies in defining the components, relationships, uncertainty labels, and decision boundaries that future prospective medicinal-chemistry programs can evaluate. Embedding such evidence earlier may help pharmaceutical discovery identify avoidable environmental liabilities while sufficient molecular and process flexibility remains to address them.