Editing Quantitative structure–activity relationship (section)

== SAR and the SAR paradox ==

The basic assumption for all molecule-based [[hypotheses]] is that similar molecules have similar activities. This principle is also called Structure–Activity Relationship ([[Structure–activity relationship|SAR]]). The underlying problem is therefore how to define a ''small'' difference on a molecular level, since each kind of activity, e.g. [[Chemical reaction|reaction]] ability, [[biotransformation]] ability, [[solubility]], target activity, and so on, might depend on another difference. Examples were given in the [[bioisosterism]] reviews by Patanie/LaVoie<ref name="pmid11848856">{{cite journal | vauthors = Patani GA, LaVoie EJ | title = Bioisosterism: A Rational Approach in Drug Design | journal = Chemical Reviews | volume = 96 | issue = 8 | pages = 3147–3176 | date = Dec 1996 | pmid = 11848856 | doi = 10.1021/cr950066q }}</ref> and Brown.<ref>{{cite book | last1 = Brown | first1 = Nathan | name-list-style = vanc | title = Bioisosteres in Medicinal Chemistry | date = 2012 | publisher = Wiley-VCH | location = Weinheim | isbn = 978-3-527-33015-7 }}</ref>

In general, one is more interested in finding strong [[Trend estimation|trends]]. Created [[Hypothesis|hypotheses]] usually rely on a [[finite set|finite]] number of chemicals, so care must be taken to avoid [[overfitting]]: the generation of hypotheses that fit training data very closely but perform poorly when applied to new data.

The ''SAR paradox'' refers to the fact that it is not the case that all similar molecules have similar activities {{Citation needed|date=April 2024}}.