Tissue biodistribution and blood clearance rates of intravenously administered carbon nanotube radiotracers

Abstract

Carbon nanotubes (CNT) are intensively being developed for biomedical applications including drug and gene delivery. Although all possible clinical applications will require compatibility of CNT with the biological milieu, their in vivo capabilities and limitations have not yet been explored. In this work, water-soluble, single-walled CNT (SWNT) have been functionalized with the chelating molecule diethylentriaminepentaacetic (DTPA) and labeled with indium ((111)In) for imaging purposes. Intravenous (i.v.) administration of these functionalized SWNT (f-SWNT) followed by radioactivity tracing using gamma scintigraphy indicated that f-SWNT are not retained in any of the reticuloendothelial system organs (liver or spleen) and are rapidly cleared from systemic blood circulation through the renal excretion route. The observed rapid blood clearance and half-life (3 h) of f-SWNT has major implications for all potential clinical uses of CNT. Moreover, urine excretion studies using both f-SWNT and functionalized multiwalled CNT followed by electron microscopy analysis of urine samples revealed that both types of nanotubes were excreted as intact nanotubes. This work describes the pharmacokinetic parameters of i.v. administered functionalized CNT relevant for various therapeutic and diagnostic applications.

Scheme 1.

Synthesis of ¹¹¹In-labeled CNT. (i) DTPA dianhydride and diisopropylethylamine (DIEA) in DMSO. (ii) Sodium citrate in H₂O. (iii) ¹¹¹InCl₃ in H₂O. Compounds 3 and 5 differ on the amount of DTPA moiety on the amino functions. Compound 3 is completely saturated with DTPA. Compound 5 presents only 60% of DTPA functionalization and 40% of free amine groups.

Fig. 1.

TEM images of single-walled (A and B) and multiwalled (C) DTPA–CNT. Highly water-soluble and homogeneously dispersed DTPA–CNT were deposited on a TEM grid for observation. (A and B) DTPA–SWNT form bundles of different length and diameters. Black arrows indicate the dimensions of SWNT bundles, each consisting of 10 and 40 tubes. The thickness of the bundles is in nm. (C) DTPA–MWNT were imaged as individual tubes with diameters ≈30–38 nm as indicated by the black arrows. (Scale bars, 200 nm.)

Fig. 2.

Biodistribution per collected gram of tissue of [¹¹¹In]DTPA–SWNT 3 (A) and [¹¹¹In]DTPA–SWNT 5 (B) after i.v. administration.

Fig. 3.

TEM images of excreted urine samples containing single- and multiwalled DTPA–CNT. The urine samples were centrifuged, and both the supernatant and the precipitate were analyzed. (A and B) DTPA–SWNT from the supernatant. (Scale bars, 500 nm.) (C–E) DTPA–MWNT into the supernatant. (F–H) DTPA–MWNT in the precipitate. (Scale bars for C–H, 100 nm.)

Fig. 4.

Blood circulation of ¹¹¹In-radiolabeled CNT. The graph represents the percentage of injected dose at different time points. i.v. injection of f-SWNT 3 (solid black line; t_1/2 = 3.52 ± 1.59) and i.v. injection of f-SWNT 5 (short dotted black line; t_1/2 = 2.99 ± 1.59) are shown. The trend presented is close to the exponential elimination profile if corrected by the standard deviation (SD). Half-life was calculated on the basis of the corresponding concentration–time data determining the elimination constant and applying the equation t_1/2 = ln2/K_el. After five half-lives, CNT are not present at all in the blood. Half-life SD was calculated by using σ = |σ_x1|/x₁+|σ_x2|/x₂ where x₁ and x₂ are the calculated percentage of injected dose at time 30 min and 24 h, and σ is the related SD.