Fraction Lipophilicity Index (FLI) was developed as a metric for assessing drug likeness of ionizable oral drugs. Considering that both log P and log D have distinct roles in drug action, the metric FLI allocates lipophilicity to a pH dependent neutral fraction of the molecule and is a weighted combination of log P and log D. It is expressed by equation: FLI = 2 x log P–│log D│. A dataset of 368 basic and acidic drugs was analyzed. Based on available % absorption data, drugs were classified into class 1 (268 drugs) and class 2 (100 drugs). The freeware MedChem Designer was used for log P and log D calculations at pH 7.4 and pH 5.5 for acids. Based on class 1, a drug-like FLI range 0–8 was defined. FLI distribution for class 2 is shifted towards significantly lower values. Comparison of FLI with rule of 5 (Ro5) shows that it leads to fewer values outside the established range than the corresponding log P violations for class 1. For class 2 it gives more alerts than Ro5 and can be considered complementary to Ro5, while it also sets lower limits to discriminate drugs with poor absorption.

分数亲脂性指数（FLI）被开发为评估可离子化口服药物的药物相似性的指标。考虑到log P和log D在药物作用中都有不同的作用，度量FLI将亲脂性分配给分子的pH依赖性中性部分，并且是log P和log D的加权组合。它是由公式表示：FLI = 2×登录P -│log d │。分析了368种基本药物和酸性药物的数据集。根据可用的％吸收数据，将药物分为1类（268种药物）和2类（100种药物）。免费软件MedChem Designer用于日志P和日志D酸在pH 7.4和pH 5.5的计算。基于第1类，定义了类似毒品的FLI范围0-8。2类的FLI分布朝着明显更低的值移动。FLI与5规则（Ro5）的比较表明，与1类的相应log P违例相比，FLI导致的值超出了已建立的范围。对于2类，它给出的警报比Ro5多，并且可以看作是对Ro5的补充。还设置了下限以区分吸收不良的药物。