Canonical signalling is where there is a generalised pathway activated by a receptor. This is seen with G-protein coupled receptors (GPCRs). This family of around 800 receptors in humans, is one of the most common drug targets.
Controlling the signalling pathways activated by GPCRs is easiest by targeting the common component - the GPCR. This is relatively easy to do with small molecule drugs, as they can bind extracellularly and control the activation of the receptor.
When an agonist binds a GPCR, it activates an intracellular G protein. This dissociates into the $$G_\alpha$$ and $$G_{\beta\gamma}$$ subunits. The $$G_{\alpha S}$$ subunit is catalytically active, and activates adenylyl cyclase. This enzyme catalyses the conversion of ATP to cAMP. cAMP subsequently activates protein kinase A, and ultimately leading to increases in gene transcription through CREB. This signalling pathway allows for the amplification of the initial signal, which can be beneficial when a low concentration of agonist is present (such as hormones).
An antagonist can bind to GPCRs as well. This prevents the receptor from being activated, inhibiting the signalling pathway. Antagonists can bind to either the allosteric site or the orthosteric site, meaning they can be competitive or non-competitive. This suppresses the cellular response to the endogenous ligand, reducing the effect of the signalling pathway.
These effects can be modulated by drugs, allowing artificial control of the signalling pathway. Being able to use small molecule drugs for this is beneficial, as they are cheap and easy to produce, and can be administered orally.