Tamoxifen-poly(ethylene glycol)-thiol gold nanoparticle conjugates: enhanced potency and selective delivery for breast cancer treatment

Abstract

The breast cancer treatment drug tamoxifen has been widely administered for more than three decades. This small molecule competes with 17beta-estradiol for binding to estrogen receptor, a hormone receptor upregulated in a majority of breast cancers, subsequently initiating programmed cell death. We have synthesized a thiol-PEGylated tamoxifen derivative that can be used to selectively target and deliver plasmonic gold nanoparticles to estrogen receptor positive breast cancer cells with up to 2.7-fold enhanced drug potency in vitro. Optical microscopy/spectroscopy, time-dependent dose-response data, and estrogen competition studies indicate that augmented activity is due to increased rates of intracellular tamoxifen transport by nanoparticle endocytosis, rather than by passive diffusion of the free drug. Both ligand- and receptor-dependent intracellular delivery of gold nanoparticles suggest that plasma membrane localized estrogen receptor alpha may facilitate selective uptake and retention of this and other therapeutic nanoparticle conjugates. Combined targeting selectivity and enhanced potency provides opportunities for both multimodal endocrine treatment strategies and adjunctive laser photothermal therapy.

Figure 1

Dark-field scattering microscopy showing ligand- and receptor- dependent intracellular targeting of breast cancer cells by gold nanoparticle conjugates. Representative dark-field scattering images of ERα(+) [MCF-7, top] and ERα(-) [MDA-MB-231, bottom] human adenocarcinoma cells incubated for 24 h with 1 μM TAM-PEG-SH AuNP (left) and PEG-SH AuNP (right) conjugates (ca. 1.2 × 10⁴ TAM-PEG-SH per AuNP).

Figure 2

Time-dependent dose-response curves for cell viability of estrogen receptor alpha positive [MCF-7] breast cancer cells incubated with equivalent concentrations of TAM-PEG-SH as a free drug (a) and as a gold nanoparticle conjugate (b). Time-dependent IC₅₀ (50% inhibitory concentration) values showing 1.3 – 2.7 times enhanced potency from the nanoparticle conjugate versus the free drug. Error bars represent standard deviation. 3.6, 1.4, 1.1 μM TAM-PEG-SH IC₅₀ (24, 36, 48 h, respectively) versus 6.4, 2.4, 1.3, 1.0, 0.88 μM TAM-PEG-SH AuNP IC₅₀ (6, 12, 24, 36, 48 h, respectively).

Figure 3

Representative dark-field scattering (red) and bright-field transmission (green) image overlays of TAM-PEG-SH AuNP competitive binding following 24 h incubation with 17β-estradiol. ERα(+) breast cancer cells [MCF-7] were incubated overnight with increasing concentrations of estrogen, followed by 24 h incubation with 10 μM tamoxifen-gold nanoparticle conjugates.

Figure 4

Suppression of TAM-PEG-SH AuNP activity by estrogen competition in ERα(+) breast cancer cells. Growth inhibition of MCF-7 cells incubated for 24 h with 10 μM TAM-PEG-SH AuNPs when previously untreated (left) and treated overnight with 10 μM 17β-estradiol (right).

Scheme 1

Synthesis of thiol-pegylated tamoxifen (TAM-PEG-SH) (a) and covalent attachment to 25 nm gold nanoparticles (AuNPs) (b).