Drug Targeting Organ-Specific Strategies

130 5 Delivery of Drugs and Antisense Oligonunucleotides to the Proximal Tubular Cell
tural requirements. When the drug moiety (AVP) was removed from Glc-O-C8-AVP, its
affinity constant was two orders of magnitude lower than the original (IC 50 of Glc-O-C7-
Me ~ 3 µM) [24]. In addition, removal of the alkyl-chain almost abolished binding (IC 50 of
Glc-O-Me > 1 mM) [24]. Thus, sugar, alkyl, and drug moieties seem to be essential for renal
targeting. However, S-glycoside alone exhibited a much higher affinity, the IC 50 of Glc-S-C7-
Me (~ 70 nM) being almost comparable with that of Glc-O-C8-AVP (Figure 5.5) [24]. The
CL uptake of Glc-S-C7-Me in the kidney was close to the renal plasma flow rate and much
higher than the CL uptake in other organs. Scatchard analysis of Glc-S-C7-Me and Glc-S-C8-
AVP revealed a Kd of 10–20 nM [24]. These results imply that Glc-S-C7-Me (octyl β-Dthioglucoside)
may be a suitable delivery vector for the kidney. It is noteworthy that this moiety
has been widely used as a detergent, but the specific binding observed occurs at a much
lower concentration than that at which it exerts a detergent effect (~ mM range) [30].
To optimize the alkyl-chain length, the effect of different numbers of methylene groups on
the CL uptake in vivo and specific binding to kidney membranes was examined. Glc-S-C5-AVP
showed a much lower CL uptake whereas Glc-S-C11-AVP had a higher CL uptake and specific
binding than Glc-S-C8-AVP [24]. By screening the inhibition potential of various types of
sugars and/or alkyl moieties, it was found that an equatorial OH group in the 4 position is essential,
while the inhibitory properties were not affected by the orientation of the OH group
at the 2 position or by its absence (Figure 5.6) [25]. The length, branching and charge in the
region of the glycoside bond within the methylene group also appeared to be important for
specific binding [25]. Thus, the alkylglycoside moiety appears to be essential for kidney targeting.
The types of therapeutic compounds that, in principle, can be delivered by conjugation
with this vector remains to be studied in detail. Until now, a marked increase in renal
CL uptake has been found only for low-molecular-weight compounds such as alkylglycosidederivatized
AVP, oxytocin, tryptamine, and 4-nitrobenz-2-oxa-1,3- diazole (NBD) [23–25].To
elucidate the size limitation of compounds to be delivered, acylated polylysines (APL) with
a range of molecular weights (Mw) were conjugated to the Glc-S-C8 moiety. The CLuptake
in kidney of Glc-S-C8-APL with a mean Mw of 17 kDa was much larger than that in other
organs, half the renal clearance being accounted for by f pGFR. Thus, molecular weight limitation
seems to be critical for the renal targeting.
Figure 5.6. Structural requirements for a renal targeting vector.