I regret to say that all my thoughts on the Bactrian king Pantaleon
have come to nothing - due to the fact that I don't read French, I
suppose. For years, I have searched all available bases on the net, but
nobody has ever mentioned this coin of Pantaleon, which belonged to the
collection of Bopearachi whose books are published in French. You can
find it on http://en.wikipedia.org/wiki/Image:KingPantaleon.jpg
The coin shows a diademed king in a pose similar to Euthydemus I, with
Zeus - the family deity of the Euthydemids - on the reverse, even
holding a miniature of Nike like on the coins of Agathocles (though
Zeus is standing on the latters coin, not sitting).
So: Pantaleon did strike coins with a portrait, and he did probably
belong to the Euthydemid dynasty. In that case he should be the brother
of Demetrius I - chronological reasons exclude him from being the son.
And with the close association between Agathocles and Pantaleon - the
latter was probably a third brother and not a usurper.