Intrinsic and Stable Conjugation of Thiolated Mesoporous Silica Nanoparticles with Radioarsenic

Abstract

The development of new image-guided drug delivery tools to improve the therapeutic efficacy of chemotherapeutics remains an important goal in nanomedicine. Using labeling strategies that involve radioelements that have theranostic pairs of diagnostic positron-emitting isotopes and therapeutic electron-emitting isotopes has promise in achieving this goal and further enhancing drug performance through radiotherapeutic effects. The isotopes of radioarsenic offer such theranostic potential and would allow for the use of positron emission tomography (PET) for image-guided drug delivery studies of the arsenic-based chemotherapeutic arsenic trioxide (ATO). Thiolated mesoporous silica nanoparticles (MSN) are shown to effectively and stably bind cyclotron-produced radioarsenic. Labeling studies elucidate that this affinity is a result of specific binding between trivalent arsenic and nanoparticle thiol surface modification. Serial PET imaging of the in vivo murine biodistribution of radiolabeled silica nanoparticles shows very good stability toward dearsenylation that is directly proportional to silica porosity. Thiolated MSNs are found to have a macroscopic arsenic loading capacity of 20 mg of ATO per gram of MSN, sufficient for delivery of chemotherapeutic quantities of the drug. These results show the great potential of radioarsenic-labeled thiolated MSN for the preparation of theranostic radiopharmaceuticals and image-guided drug delivery of ATO-based chemotherapeutics.

Keywords: As-72; As-76; As-77; arsenic trioxide; image-guided drug delivery; positron emission tomography; radioarsenic; radiolabeling.

Figure 1

Transmission electron microscopy images of three types of silica nanoparticles used for radioarsenic labeling and affinity studies.

Figure 2

Stability of *As(III) oxidation state toward auto-oxidation to *As(V) in 0.1 M HEPES, 0.5 M HA, 25 mM EDTA at various pH, temperatures. Plots show the changes of percent of *As(III) (blue squares) and *As(V) (red circles) in solutions at pH 5.5 (solid lines), pH 7.5 (dotted lines), and pH 9 (dashed lines) at (a) 21 °C and (b) 80 °C. While uncertainties were not quantified in these radio-TLC studies, the methods resulted in high uncertainty estimated to be ~15% in solutions containing nearly 0% *As(III) and nearly 100% *As(V).

Figure 3

Representative labeling results of MSN(3 nm pores)–SH (600 nmol [–SH]/mL) and MSN(3 nm pores)–NH₂ (520 nmol [–NH₂]/mL) with *As(OH)₃ in 0.1 M HEPES, 70 mM HA, 3 mM EDTA solutions. (a) Labeling temperature was set to be 21 °C. (b) Labeling temperature was set to be 80 °C. (c) Labeling results of MSN(3 nm pores)–SH (600 nmol [–SH]/mL) with *As(OH)₃ in 0.1 M HEPES, 70 mM HA, 3 mM EDTA solutions, under varied pH conditions, pH 5.5, 7.5, and 9, at 21 °C. (d) Labeling results of 490 nmol (–SH)/mL MSN(3 nm pores)–SH with *AsO(OH)₃ in 0.1 M HEPES, 0.14 M HA, 7 mM EDTA solutions at pH 7.5, 21 °C. While uncertainties were not quantified in these radio-TLC studies, the methods resulted in high uncertainty estimated to be ~15% in solutions containing nearly 0% *As(III) or *As-MSN or 100% *As(V).

Figure 4

Labeling results of various concentrations of MSN(3 nm pores)–SH with *As(OH)₃ in 0.1 M HEPES, 70 mM HA, 3 mM EDTA solutions at pH 9. While uncertainties were not quantified in these radio-TLC studies, the methods resulted in high uncertainty estimated to be ~15% in solutions containing nearly 0% *As-MSN.

Figure 5

Stability of centrifugation/decantation-isolated *As-MSN (3 nm pores) reformulated in whole mouse serum and incubated at 21 and 37 °C. While uncertainties were not quantified in these radio-TLC studies, the methods resulted in high uncertainty estimated to be ~15% in solutions containing nearly 0% *As(III)/*As(V) and nearly 100% *As-MSN (3 nm pores).

Figure 6

Serial MIP PET images of healthy BALB/c mice following iv administration of (a) free *As, (b) *As-MSN (3 nm pores), (c) *As-dSiO₂, and (d) *As-MSN (5 nm pores).

Figure 7

Average PET-quantified percent injected dose per gram (%ID/g) in healthy mice (n = 3) of iv administered free *As (a), *As-MSN(3 nm pores), *As-dSiO₂, and *As-MSN(5 nm pores) in the liver and bladder (b, c) as a function of time postinjection. (d) Ex vivo biodistribution of *As-dSiO₂, *As-MSN(3 nm pores), and *As-MSN(5 nm pores) 7 days postinjection in healthy mice (n = 3).

Figure 8

Results of ATO loading capacity experiments plotted as equilibrium ATO mass bound to MSN(5 nm pores) in mg ATO per g of MSN as a function of ATO mass left unbound in solution in mg ATO per mL of solution. Uncertainties in [ATO]_eq,MSN quantified from triplicate MP-AES measurements were measured to be less than or equal to ~1 mg/g. Line drawn to guide the eye.