Free Serum and Salivary Drug Measurement for the PIs

Some two and a half years ago, we developed an Institutional Review Board (IRB)-approved protocol for the assessment of free drug concentrations by measuring both the saliva and the free drug in plasma utilizing the same tandem mass spectrometric methods for plasma or serum measurement as reported earlier. Our specified sample is either plasma or serum, as the methods we published previously work equally well on both. We, however, inject a bigger sample for the free drug measurement (50 vs. 10 ^L for plasma/serum). Free drug concentrations can be measured by using either equilibrium dialysis or, more commonly, devices with molecular cutoff filters such as the Amicon Centrifree micropartition system (Millipore Corporation, Bedford, MA) or the Worthington Diagnostics "ultrafree" system (Worthington, Jacksonville, FL). Plasma ultrafiltrate is obtained by centrifuging 1 mL of plasma in Amicon system and

Free=0.419 Total+144 n=20 r=0.949 Syx=904.9

15000

0 10000 20000 30000

Serum Total ng/mL

Fig. 1. Nevirapine seram total versus seram free (free = 0.419 x total + 144; n = 20; r = 0.949; syx = 904.9).

0 10000 20000 30000

Serum Total ng/mL

Sal=1.234 Free+584 n=17 r=0.969 Syx=954.5

Serum Free ng/mL

Fig. 2. Nevirapine serum free versus saliva (sal = 1.234 x free + 584; n = 17; r = 0.969; syx = 954.5).

Serum Free ng/mL

Fig. 2. Nevirapine serum free versus saliva (sal = 1.234 x free + 584; n = 17; r = 0.969; syx = 954.5).

then deproteinizing as described for serum/plasma previously (33-35). Figures 1-3 show the data for nevirapine (an NNRTI). As can be seen, the salivary concentration correlates very well with both serum-total and serum-free concentration. Figures 4 and 5 show the correlations for stavudine (an NRTI). Here the correlation between serum free and serum total is excellent, with a less good correlation between saliva and serum-total drug concentration. Finally, Figs 6 and 7 show the correlations between serum-total and saliva concentrations and between serum-total and serum-free drug concentrations and are adequate at best for abacavir (an NRTI). This is exciting as it opens up the possibility of salivary monitoring in place of plasma monitoring to

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