Pharmacokinetic and biodistribution studies of a bone-targeting drug delivery system based on N-(2-hydroxypropyl)methacrylamide copolymers

Abstract

Osteotropicity of novel bone-targeted HPMA copolymer conjugates has been demonstrated previously with bone histomorphometric analysis. The pharmacokinetics and biodistribution of this delivery system were investigated in the current study with healthy young BALB/c mice. The 125I-labeled bone-targeted and control (nontargeted) HPMA copolymers were administered intravenously to mice, and their distribution to different organs and tissues was followed using gamma counter and single photon emission computed tomography (SPECT). Both the invasive and noninvasive data further confirmed that the incorporation of D-aspartic acid octapeptide (D-Asp8) as bone-targeting moiety could favorably deposit the HPMA copolymers to the entire skeleton, especially to the high bone turnover sites. To evaluate the influence of molecular weight, three fractions (Mw of 24, 46, and 96 kDa) of HPMA copolymer-D-Asp8 conjugate were prepared and evaluated. Higher molecular weight of the conjugate enhanced the deposition to bone due to the prolonged half-life in circulation, but it weakened the bone selectivity. A higher content of bone-targeting moiety (D-Asp8) in the conjugate is desirable to achieve superior hard tissue selectivity. Further validation of the bone-targeting efficacy of the conjugates in animal models of osteoporosis and other skeletal diseases is needed in the future.

Figure 1

Structure of Tyr-labeled HPMA copolymer - D-Asp₈ conjugates and of the peptide Tyr-D-Asp₈.

Figure 2

Effect of molecular weight on the biodistribution of Tyr-labeled HPMA copolymer - D-Asp₈ conjugates (P-D-Asp₈). HPMA copolymers (no peptide) and Tyr-D-Asp₈ were used as controls. The ¹²⁵I-labeled samples were administered intravenously to BALB/c mice and the biodistribution analyzed at time intervals indicated. (A) 15 min, (B) 30 min, (C) 1 h, (D) 4 h, (E) 24 h. P < 0.05 except for P-D-Asp₈ (24 kDa and 46 kDa) in the kidneys and bone of (C); (F) 24 h – comparison of targeted and non-targeted conjugates. T/F×2 represents two sets of tibia and femur. The accumulation in the bone was significantly higher (p < 0.05) for all three P-D-Asp₈ as compared to their respective P_control. The targeting strategy did not result in significantly higher accumulation in any of the other organs with any of the different molecular weight conjugates except for P-D-Asp₈ 96 kDa, which showed a significantly higher accumulation in the liver and spleen as compared to P_control of the similar molecular weight.

Figure 3

Planar and tomographic images of BALB/c mice 24 h after i.v. administration of Tyr-labeled HPMA copolymer – D-Asp₈ conjugates and of Tyr-D-Asp₈ recorded by single photon emission computed tomography.

Figure 4

Blood clearance of ¹²⁵I-labeled HPMA copolymer-D-Asp₈ conjugates after intravenous administration to BALB/c mice. Radioactivity in the bloodstream (% injected dose) is shown for conjugates of different molecular weight and for Tyr-D-Asp₈.

Figure 5

Histomorphometric analysis of long bones from BALB/c mice intravenously injected with P_control-FITC (M_w = 68 kDa, PDI = 1.5, [FITC] = 1.14 × 10⁻⁴ mol/g) and P-D-Asp₈-FITC (M_w = 68 kDa, PDI = 1.5, [D-Asp₈] = 1.77 × 10⁻⁴ mol/g, [FITC] = 1.05 × 10⁻⁴ mol/g). (1) Section through the mid-diaphyseal shaft illustrating the retention of the P_control-FITC in the marrow sinusoids (arrows); (2) Section through the mid-diaphyseal shaft illustrating the uptake of P-D-Asp₈-FITC on the endocortical surface (arrows). B = bone, M = marrow. Magnification 15 ×.