Commenting on the antiquity of Psa 110, John Day wrote:
[S]ince the time of Gunkel (1926; cf. Gunkel and Begrich 1928-33, ET 1998) the following psalms have typically been regarded as royal in the sense of relating to kings of the Davidic monarchy and have accordingly been regarded as pre-exilic (or sometimes in the case of Psalm 89, exilic): Psalms 2, 18, 20, 21, 45, 72, 89, 101, 110, 132 and 144.1-11. (Ps. 144.12-15 is rightly seen as a later addition, as it contains late language as well as distinctive content) . . . Ps. 110 has been seen as relating specifically to Simon Maccabaeus by other occasional scholars in recent years (cf. Gerleman 1981; Donner 1991; Astour 1992:685). However, it is most improbable that there are any Maccabaean psalms in the Psalter and, has been noted, Ps. 110 bears signs of antiquity (cf. further Hardy 1945), and it seems most natural in the light of v. 4's attribution to the king of a priesthood after the order o the pre-Israelite, Jebusite ruler Melchizedek 9cf. Gen. 14:18-20) to suppose that this is a very early psalm, perhaps even Davidic, fusing the Israelite and Jebusite royal ideologies. That the title 'high priest of God Most High' (i.e. El-Elyon) should later be appropriated by the Hasmonaean kings (cf. Josephus, Ant. 16.153; Ass. Mos. 6.1) from Gen. 14:18 is entirely understandable, since taken with Ps. 110.4 this could be used to justify their non-Zadokite status Moreover, the old view that Ps. 110 contains an acrostic relating to Simon Maccabaeus, revived by Treves and some others, has been thoroughly refuted by Bowker 1967 (John Day, "How Many Pre-Exilic Psalms Are There?" in John Day, ed. In Search of Pre-Exilic Israel [JSOT Supplement Series 406; London: T&T Clark International, 2004], 225-50, here, 226, p. 226 n. 1)
That Psa 110 is directed, not to the then-future Messiah, Jesus, merely, but also to the Davidic Kings in ancient Israel, Eric Mason wrote the following:
Psalm 110:4
Melchizedek is evoked in this royal psalm addressed to a ruler of the Davidic dynasty, perhaps as part of an enthronement or other similar ritual. The relevant verse, Ps 110:4 (Ps 109:4 LXX), reads:
MT: נִשְׁבַּ֤ע יְהוָ֙ה׀ וְלֹ֥א יִנָּחֵ֗ם אַתָּֽה־כֹהֵ֥ן לְעוֹלָ֑ם עַל־דִּ֜בְרָתִ֗י מַלְכִּי־צֶֽדֶק
LXX: ὤμοσεν κύριος καὶ οὐ μεταμεληθήσεται
σὺ εἶ ἱερεὺς εἰς τὸν αἰῶνα
κατὰ τὴν τάξιν Μελχισεδεκ
NRSV: The LORD has sworn and will not change his mind,
“You are a priest forever
according to the order of Melchizedek.”
“You are a priest forever
according to the order of Melchizedek.”
An eternal priesthood somehow related to that of Melchizedek is bestowed on the king. (Eric F. Mason, “You Are a Priest Forever”: Second Temple Jewish Messianism and the Priestly Christology of the Epistle to the Hebrews [Studies on the Text of the Desert of Judah 74; Atlanta: SBL Press, 2008], 143)
