Cetirizine is an antihistamine. Antihistamines block the release of PGD2 as well as numerous other inflammatory markers from mast cells (the primary storage site of PGD2 in skin) and other cells as well.
For example, with desloratadine, which I think is better due to better stability in alcohol, and a long binding time to receptors:
In vitro studies have shown that desloratadine inhibits the release or generation of multiple inflammatory mediators, including IL-4, IL-6, IL-8, IL-13, PGD(2), leukotriene C(4), tryptase, histamine, and the TNF-alpha-induced chemokine RANTES.
https://www.ncbi.nlm.nih.gov/pubmed/11295678
So basically these antihistamines are working as very effective local anti-inflammatories. And given that one of the most characteristic components of male balding is "itch" and itch is directly related in many cases to histamines, an anti-histamine may be the exact type of anti-inflammatory we need.
Their results were similar to minoxidil in power. So it wasn't a minor effect. It was actually pretty damn good.
I've ordered some desloratadine powder from Alibaba and I will be adding it to my formulations. Unlike minoxidil, a topical antihistamine should be free of pro-aging or collagen damaging side effects. If it lets me stop minoxidil, I will consider it a great gift.