Which class of agents is least likely to be used as premedication for CINV in Mr. Rylan's regimen?

The class of agents least likely to be used as premedication for chemotherapy-induced nausea and vomiting (CINV) is beta-blockers. Beta-blockers are primarily used in the management of cardiovascular conditions, such as hypertension and arrhythmias, and are not indicated for the prevention or treatment of nausea and vomiting.

In contrast, 5HT3 antagonists, prokinetic agents, and neurokinin 1 antagonists are all specifically designed to target the pathways involved in CINV. 5HT3 antagonists are widely used for their effectiveness in blocking serotonin receptors, which helps to mitigate nausea and vomiting. Prokinetic agents work by enhancing gastrointestinal motility and can assist in preventing nausea. Neurokinin 1 antagonists are also effective in blocking substance P, which is involved in the vomiting reflex, making them valuable in the prevention of delayed nausea and vomiting associated with certain chemotherapy regimens.

Using beta-blockers for this purpose would not only be ineffective but also potentially inappropriate given their pharmacological profile. Therefore, they are the least likely to be used as premedication for CINV in a chemotherapy treatment plan.