Colloidal Drug Aggregate Stability in High Serum Conditions and Pharmacokinetic Consequence

Colloidal drug aggregates have been a nuisance in drug screening, yet, because they inherently comprise drug-rich particles, they may be useful in vivo if issues of stability can be addressed. As the first step toward answering this question, we optimized colloidal drug aggregate formulations using a fluorescence-based assay to study fulvestrant colloidal formation and stability in high (90%) serum conditions in vitro. We show, for the first time, that the critical aggregation concentration of fulvestrant depends on media composition and increases with serum concentration. Excipients, such as polysorbate 80, stabilize fulvestrant colloids in 90% serum in vitro for over 48 h. Using fulvestrant and an investigational pro-drug, pentyloxycarbonyl-( p-aminobenzyl) doxazolidinylcarbamate (PPD), as proof-of-concept colloidal formulations, we demonstrate that the in vivo plasma half-life for stabilized colloids is greater than their respective monomeric forms. These studies demonstrate the potential of turning the nuisance of colloidal drug aggregation into an opportunity for drug-rich formulations.

Photoactivatable Prodrug of Doxazolidine Targeting Exosomes

Natural lipid nanocarriers, exosomes, carry cell-signaling materials such as DNA and RNA for intercellular communications. Exosomes derived from cancer cells contribute to the progression and metastasis of cancer cells by transferring oncogenic signaling molecules to neighboring and remote premetastatic sites. Therefore, applying the unique properties of exosomes for cancer therapy has been expected in science, medicine, and drug discovery fields. Herein, we report that an exosome-targeting prodrug system, designated MARCKS-ED-photodoxaz, could spatiotemporally control the activation of an exquisitely cytotoxic agent, doxazolidine (doxaz), with UV light. The MARCKS-ED peptide enters a cell by forming a complex with the exosomes in situ at its plasma membrane and in the media. MARCKS-ED-photodoxaz releases doxaz under near-UV irradiation to inhibit cell growth with low nanomolar IC₅₀ values. The MARCKS-ED-photodoxaz system targeting exosomes and utilizing photochemistry will potentially provide a new approach for the treatment of cancer, especially for highly progressive and invasive metastatic cancers.

Leveraging Colloidal Aggregation for Drug-Rich Nanoparticle Formulations

While limited drug loading continues to be problematic for chemotherapeutics formulated in nanoparticles, we found that we could take advantage of colloidal drug aggregation to achieve high loading when combined with polymeric excipients. We demonstrate this approach with two drugs, fulvestrant and pentyl-PABC doxazolidine (PPD; a prodrug of doxazolidine, which is a DNA cross-linking anthracycline), and two polymers, polysorbate 80 (UP80) and poly(d,l-lactide-co-2-methyl-2-carboxytrimethylene carbonate)-graft-poly(ethylene glycol) (PLAC-PEG; a custom-synthesized, self-assembling amphiphilic polymer). In both systems, drug-loaded nanoparticles had diameters < 200 nm and were stable for up to two days in buffered saline solution and for up to 24 h in serum-containing media at 37 °C. While colloidal drug aggregates alone are typically unstable in saline and serum-containing media, we attribute the colloid stability observed herein to the polymeric excipients and consequent decreased protein adsorption. We expect this strategy of polymer-stabilized colloidal drug aggregates to be broadly applicable in delivery formulations.

Carboxylesterase-2 is a highly sensitive target of the antiobesity agent orlistat with profound implications in the activation of anticancer prodrugs

Orlistat has been the most used anti-obesity drug and the mechanism of its action is to reduce lipid absorption by inhibiting gastrointestinal lipases. These enzymes, like carboxylesterases (CESs), structurally belong to the α/β hydrolase fold superfamily. Lipases and CESs are functionally related as well. Some CESs (e.g., human CES1) have been shown to hydrolyze lipids. This study was designed to test the hypothesis that orlistat inhibits CESs with higher potency toward CES1 than CES2, a carboxylesterase with little lipase activity. Liver microsomes and recombinant CESs were tested for the inhibition of the hydrolysis of standard substrates and the anticancer prodrugs pentyl carbamate of p-aminobenzyl carbamate of doxazolidine (PPD) and irinotecan. Contrary to the hypothesis, orlistat at 1 nM inhibited CES2 activity by 75% but no inhibition on CES1, placing CES2 one of the most sensitive targets of orlistat. The inhibition varied among some CES2 polymorphic variants. Pretreatment with orlistat reduced the cell killing activity of PPD. Certain mouse but not rat CESs were also highly sensitive. CES2 is responsible for the hydrolysis of many common drugs and abundantly expressed in the gastrointestinal track and liver. Inhibition of this carboxylesterase probably presents a major source for altered therapeutic activity of these medicines if co-administered with orlistat. In addition, orlistat has been linked to various types of organ toxicities, and this study provides an alternative target potentially involved in these toxicological responses.

Synthesis and biological characterization of protease-activated prodrugs of doxazolidine

Doxazolidine (doxaz) is a new anthracycline anticancer agent. While structurally similar to doxorubicin (dox), doxaz acts via a distinct mechanism to selectively enhance anticancer activity over cardiotoxicity, the most significant clinical impediment to successful anthracycline treatment. Here, we describe the synthesis and characterization of a prodrug platform designed for doxaz release mediated by secreted proteolytic activity, a common association with invasiveness and poor prognosis in cancer patients. GaFK-Doxaz is hydrolyzable by the proteases plasmin and cathepsin B, both strongly linked with cancer progression, as well as trypsin. We demonstrate that activation of GaFK-Doxaz releases highly potent doxaz that powerfully inhibits the growth of a wide variety of cancer cells (average IC(50) of 8 nM). GaFK-Doxaz is stable in human plasma and is poorly membrane permeable, thereby limiting activation to locally secreted proteolytic activity and reducing the likelihood of severe side effects.

Abstract A29: Bcl-2 overexpression provides short-term resistance to doxorubicin-DNA adducts but does not protect against longer-term treatments in HL-60 cells

The anthracycline anticancer agent doxorubicin functions primarily as a topoisomerase II inhibitor but forms more cytotoxic DNA adducts in the presence of formaldehyde. For this reason, the efficacy of doxorubicin can be greatly enhanced with the co-administration of formaldehyde-releasing prodrugs such as AN-9. The combination of doxorubicin with AN-9 results in DNA adduct formation and synergistic apoptosis in HL-60 leukemic cells after 6 hr. Under the same conditions HL-60 cells overexpressing the anti-apoptotic protein Bcl-2 (HL-60/Bcl-2 cells) are completely resistant to doxorubicin/AN-9 treatments. This resistance can be overcome with the use of the small molecule Bcl-2/Bcl-xL/BCL-W inhibitor ABT-737.

While it appears that Bcl-2 overexpression confers resistance and thus may limit the therapeutic potential of doxorubicin-DNA adduct forming treatments, this is shown to be only a short-term effect. The use of a DNA adduct forming doxorubicin-formaldehyde conjugate, doxazolidine, produced similar results to the doxorubicin/AN-9 combination with HL-60/Bcl-2 cells being resistant to the drug after 4 hr treatment. This resistance was maintained up to 12 hr treatment, however, after 18 hr the HL-60/Bcl-2 cells started to display classical hallmarks of apoptosis including DNA fragmentation, and chromatin condensation which increased over time (18–36 hr). These results suggest that the Bcl-2 protection is only short-lived and that over time the resistance is overcome and the cells die via classical apoptosis. The exact mechanisms involved in overcoming this resistance in HL-60/Bcl-2 cells are currently being investigated and may involve changes in expression of Bcl-2 family proteins, or Bcl-2 interactions with pro-apoptotic proteins which tip the balance in favour of apoptosis. These findings imply that the damage caused by these DNA adduct forming compounds may be sufficient to overcome Bcl-2 mediated resistance observed in tumors in vivo.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A29.

Aptamer-based multiplexed proteomic technology for biomarker discovery

BACKGROUND

The interrogation of proteomes ("proteomics") in a highly multiplexed and efficient manner remains a coveted and challenging goal in biology and medicine.

METHODOLOGY/PRINCIPAL FINDINGS

We present a new aptamer-based proteomic technology for biomarker discovery capable of simultaneously measuring thousands of proteins from small sample volumes (15 µL of serum or plasma). Our current assay measures 813 proteins with low limits of detection (1 pM median), 7 logs of overall dynamic range (~100 fM-1 µM), and 5% median coefficient of variation. This technology is enabled by a new generation of aptamers that contain chemically modified nucleotides, which greatly expand the physicochemical diversity of the large randomized nucleic acid libraries from which the aptamers are selected. Proteins in complex matrices such as plasma are measured with a process that transforms a signature of protein concentrations into a corresponding signature of DNA aptamer concentrations, which is quantified on a DNA microarray. Our assay takes advantage of the dual nature of aptamers as both folded protein-binding entities with defined shapes and unique nucleotide sequences recognizable by specific hybridization probes. To demonstrate the utility of our proteomics biomarker discovery technology, we applied it to a clinical study of chronic kidney disease (CKD). We identified two well known CKD biomarkers as well as an additional 58 potential CKD biomarkers. These results demonstrate the potential utility of our technology to rapidly discover unique protein signatures characteristic of various disease states.

CONCLUSIONS/SIGNIFICANCE

We describe a versatile and powerful tool that allows large-scale comparison of proteome profiles among discrete populations. This unbiased and highly multiplexed search engine will enable the discovery of novel biomarkers in a manner that is unencumbered by our incomplete knowledge of biology, thereby helping to advance the next generation of evidence-based medicine.

Preclinical efficacy of a carboxylesterase 2-activated prodrug of doxazolidine

Doxazolidine (Doxaz) is a functionally distinct formaldehyde conjugate of doxorubicin (Dox) that induces cancer cell death in Dox-sensitive and resistant cells. Pentyl PABC-Doxaz (PPD) is a prodrug of Doxaz that is activated by carboxylesterase 2 (CES2), which is expressed by liver, non-small-cell lung, colon, pancreatic, renal, and thyroid cancer cells. Here, we demonstrate that in two murine models, PPD was effective at slowing tumor growth and demonstrated markedly reduced cardiotoxic and nephrotoxic effects, as well as better tolerance, relative to Dox. Hepatotoxicity, consistent with liver expression of the murine CES2 homologue, was induced by PPD. Unlike irinotecan, a clinical CES2-activated prodrug, PPD produced no visible gastrointestinal damage. Finally, we demonstrate that cellular response to PPD may be predicted with good accuracy using CES2 expression and Doxaz sensitivity, suggesting that these metrics may be useful as clinical biomarkers for sensitivity of a specific tumor to PPD treatment.

DNA repair in response to anthracycline-DNA adducts: a role for both homologous recombination and nucleotide excision repair

Doxorubicin, a widely used anthracycline anticancer agent, acts as a topoisomerase II poison but can also form formaldehyde-mediated DNA adducts. This has led to the development of doxorubicin derivatives such as doxoform, which can readily form adducts with DNA. This work aimed to determine which DNA repair pathways are involved in the recognition and possible repair of anthracycline-DNA adducts. Cell lines lacking functional proteins involved in each of the five main repair pathways, mismatch repair (MMR), base excision repair (BER), nucleotide excision repair (NER), homologous recombination (HR) and non-homologous end-joining (NHEJ) were examined for sensitivity to various anthracycline adduct-forming treatments. The treatments used were doxorubicin, barminomycin (a model adduct-forming anthracycline) and doxoform (a doxorubicin-formaldehyde conjugate). Cells with deficiencies in MMR, BER and NHEJ were equally sensitive to adduct-forming treatments compared to wild type cells and therefore these pathways are unlikely to play a role in the repair of these adducts. Some cells with deficiencies in the NER pathway (specifically, those lacking functional XPB, XPD and XPG), displayed tolerance to adducts induced by both barminomycin and doxoform and also exhibited a decreased level of apoptosis in response to adduct-forming treatments. Conversely, two HR deficient cell lines were shown to be more sensitive to barminomycin and doxoform than HR proficient cells, indicating that this pathway is also involved in the repair response to anthracycline-DNA adducts. These results suggest an unusual damage response pathway to anthracycline adducts involving both NER and HR that could be used to optimise cancer therapy for tumours with either high levels of NER or defective HR. Tumours with either of these characteristics would be predicted to respond particularly well to anthracycline-DNA adduct-forming treatments.

Doxazolidine induction of apoptosis by a topoisomerase II independent mechanism

The mechanism of doxorubicin is compared with that of doxazolidine, a doxorubicin-formaldehyde conjugate. The IC(50) for growth inhibition of 67 human cancer cell lines, but not cardiomyocytes, is 32-fold lower with doxazolidine than with doxorubicin. Growth inhibition by doxazolidine correlates better with growth inhibition by DNA cross-linking agents than with growth inhibition by doxorubicin. Doxorubicin induces G2/M arrest in HCT-116 colon cancer cells and HL-60 leukemia cells through a well-documented topoisomerase II dependent mechanism. Doxazolidine fails to induce a G2/M arrest in HCT-116 cells but induces apoptosis 4-fold better than doxorubicin. The IC(50) for doxazolidine growth inhibition of HL-60/MX2 cells, a topoisomerase II deficient derivative of HL-60 cells, is 1420-fold lower than the IC(50) for doxorubicin, and doxazolidine induces apoptosis 15-fold better. Further, doxazolidine has little effect in a topoisomerase II activity assay. These data indicate that doxorubicin and doxazolidine induce apoptosis via different mechanisms and doxazolidine cytotoxicity is topoisomerase II independent.

Design, synthesis, and preliminary evaluation of doxazolidine carbamates as prodrugs activated by carboxylesterases

The synthesis and tumor cell growth inhibition by doxazolidine carbamate prodrugs are reported. The carbamates were designed for selective hydrolysis by one or more human carboxylesterases to release doxazolidine (Doxaz), the formaldehyde-oxazolidine of doxorubicin that cross-links DNA to trigger cell death. Simple butyl and pentyl, but not ethyl, carbamate prodrugs inhibited the growth of cancer cells that overexpress carboxylesterase CES1 (hCE1) and CES2 (hiCE). Relative CES1 and CES2 expression levels were determined by reverse transcription of the respective mRNAs, followed by polymerase chain reaction amplification. More complex structures with a p-aminobenzyl alcohol (PABA) self-eliminating spacer showed better growth inhibition (IC50=50 nM for Hep G2 liver cancer cells) while exhibiting reduced toxicity toward rat cardiomyocytes, relative to the parent drug doxorubicin. Pentyl 4-(N-doxazolidinylcarbonyloxymethyl)phenylcarbamate, the lead compound for further investigation, appears to be activated in Hep G2 cells that express both CES1 and CES2.

Doxazolidine, a Proposed Active Metabolite of Doxorubicin That Cross-links DNA

A crystal structure establishes doxoform as a dimeric formaldehyde conjugate of the oxazolidine of doxorubicin. Doxoform is a prodrug of doxazolidine, a monomeric doxorubicin formaldehyde-oxazolidine. Both doxoform and doxazolidine inhibit the growth of cancer cells at 1-4 orders of magnitude lower concentration than doxorubicin. They also inhibit the growth of cancer cells better than doxsaliform, a prodrug for an acyclic doxorubicin-formaldehyde conjugate. Doxoform rapidly hydrolyzes to doxazolidine, which then hydrolyzes to doxorubicin with a half-life of 3 min in human serum at 37 degrees C. Both doxoform and doxazolidine are taken up by multidrug-resistant MCF-7/Adr cells 3- to 4-fold better than doxorubicin. A molecular model suggests that doxazolidine can cross-link DNA by direct reaction with a G-base in a tautomeric form with synchronous ring opening of the oxazolidine. These results point to doxoform being a prodrug for doxazolidine that is the reactive species that directly cross-links DNA.

Doxorubicin-formaldehyde conjugates targeting alphavbeta3 integrin

We have reported the synthesis and biological evaluation of a prodrug to a doxorubicin active metabolite. Under physiologic conditions, release of the active metabolite, a conjugate of doxorubicin with formaldehyde, occurs with a half-life of 1 hour. To direct this prodrug to tumor, we designed two conjugates of the prodrug, doxsaliform, with the alphavbeta3-targeting peptides, CDCRGDCFC (RGD-4C) and cyclic-(N-Me-VRGDf) (Cilengitide). We now report the synthesis of these doxsaliform-peptide conjugates and their evaluation using MDA-MB-435 cancer cells. A hydroxylamine ether tether was used to attach 5''-formyldoxsaliform to RGD-4C in its acyclic form via an oxime functional group. The construct acyclic-RGD-4C-doxsaliform showed good binding affinity for alphavbeta3 in the vitronection cell adhesion assay (IC50 = 10 nmol/L) and good growth inhibition of MDA-MB-435 breast cancer cells (IC50 = 50 nmol/L). In its bicyclic forms, RGD-4C showed less affinity for alphavbeta3 and significantly less water solubility. Cyclic-(N-Me-VRGDf) was modified by substitution of D-4-aminophenylalanine for D-phenylalanine to provide a novel attachment point for doxsaliform. The conjugate, cyclic-(N-Me-VRGDf-NH)-doxsaliform, maintained a high affinity for alphavbeta3 (IC50 = 5 nmol/L) in the vitronectin cell adhesion assay relative to the peptide bearing only the tether (0.5 nmol/L). The IC50 for growth inhibition of MDA-MB-435 cells was 90 nmol/L. Flow cytometry and growth inhibition experiments suggest that the complete drug construct does not penetrate through the plasma membrane, but the active metabolite does on release from the targeting group. These drug conjugates could have significantly reduced side effects and are promising candidates for in vivo evaluation in tumor-bearing mice.

Doxorubicin-formaldehyde conjugates targeting αvβ3 integrin

We have reported the synthesis and biological evaluation of a prodrug to a doxorubicin active metabolite. Under physiologic conditions, release of the active metabolite, a conjugate of doxorubicin with formaldehyde, occurs with a half-life of 1 hour. To direct this prodrug to tumor, we designed two conjugates of the prodrug, doxsaliform, with the αvβ3-targeting peptides, CDCRGDCFC (RGD-4C) and cyclic-(N-Me-VRGDf) (Cilengitide). We now report the synthesis of these doxsaliform-peptide conjugates and their evaluation using MDA-MB-435 cancer cells. A hydroxylamine ether tether was used to attach 5″-formyldoxsaliform to RGD-4C in its acyclic form via an oxime functional group. The construct acyclic-RGD-4C-doxsaliform showed good binding affinity for αvβ3 in the vitronection cell adhesion assay (IC50 = 10 nmol/L) and good growth inhibition of MDA-MB-435 breast cancer cells (IC50 = 50 nmol/L). In its bicyclic forms, RGD-4C showed less affinity for αvβ3 and significantly less water solubility. Cyclic-(N-Me-VRGDf) was modified by substitution of d-4-aminophenylalanine for d-phenylalanine to provide a novel attachment point for doxsaliform. The conjugate, cyclic-(N-Me-VRGDf-NH)-doxsaliform, maintained a high affinity for αvβ3 (IC50 = 5 nmol/L) in the vitronectin cell adhesion assay relative to the peptide bearing only the tether (0.5 nmol/L). The IC50 for growth inhibition of MDA-MB-435 cells was 90 nmol/L. Flow cytometry and growth inhibition experiments suggest that the complete drug construct does not penetrate through the plasma membrane, but the active metabolite does on release from the targeting group. These drug conjugates could have significantly reduced side effects and are promising candidates for in vivo evaluation in tumor-bearing mice.

Antiestrogen Binding Site and Estrogen Receptor Mediate Uptake and Distribution of 4-Hydroxytamoxifen-Targeted Doxorubicin−Formaldehyde Conjugate in Breast Cancer Cells

The anthracycline antitumor drug, doxorubicin (DOX), has long been used as a broad spectrum chemotherapeutic. The literature now documents the role of formaldehyde in the cytotoxic mechanism, and anthracycline-formaldehyde conjugates possess substantially enhanced activity in vitro and in vivo. We have recently reported the design, synthesis, and preliminary evaluation of a doxorubicin-formaldehyde conjugate targeted, via 4-hydroxytamoxifen, to the estrogen receptor (ER) and antiestrogen binding site (AEBS), which are commonly present in breast cancer cells. The lead targeted doxorubicin-formaldehyde conjugate, called DOX-TEG-TAM, was found to possess superior cell growth inhibition characteristics relative to clinical doxorubicin and an untargeted control conjugate, especially in ER-negative, multidrug resistant MCF-7/Adr cells. The enhanced activity in the absence of estrogen receptor raised the possibility that targeting was also mediated via AEBS. Fluorescence microscopy of an ER-negative, AEBS-positive cell line as a function of time showed initial DOX-TEG-TAM localization in cytosol, in contrast to initial DOX and untargeted doxorubicin-formaldehyde conjugate localization in the nucleus. DOX-TEG-TAM was taken up by four AEBS-positive cell lines to a greater extent than doxorubicin and an untargeted doxorubicin-formaldehyde conjugate. Of the four cell lines, three were ER negative. DOX-TEG-TAM uptake was inhibited in a dose-dependent manner by the presence of a competing AEBS ligand. DOX-TEG-TAM retains 60% of the affinity of 4-hydroxytamoxifen for AEBS. DOX-TEG-TAM was also taken up by the AEBS-negative, ER-positive cancer cell line Rtx-6; with these cells uptake was inhibited in a dose-dependent manner by the ER ligand, estradiol. The data support the hypothesis that uptake of 4-hydroxytamoxifen targeted doxorubicin-formaldehyde conjugate is mediated by both the antiestrogen binding site and estrogen receptor.

Studies of Targeting and Intracellular Trafficking of an Anti-Androgen Doxorubicin−Formaldehyde Conjugate in PC-3 Prostate Cancer Cells Bearing Androgen Receptor-GFP Chimera

The synthesis of a doxorubicin-formaldehyde conjugate bound to the nonsteroidal anti-androgen cyanonilutamide, via a cleavable tether, and binding of the construct to cell free androgen receptor (AR) as a function of tether design were previously reported. Cyanonilutamide bearing a linear alkyne tether bound to the AR better than other designs. Fluorescence microscopy studies of binding of the lead targeted drug, as well as various tethered cyanonilutamides, to the AR and subsequent trafficking of the resulting AR complex in live PC3 prostate cancer cells transfected with AR-green fluorescent protein (GFP) chimera are now described. Cyanonilutamide and cyanonilutamide bonded to a linear alkyne tether caused translocation of AR-GFP to the nucleus. In general, the ability of tethered cyanonilutamides to cause translocation paralleled their binding affinity for the AR. However, a noncleavable form of the lead cyanonilutamide-doxorubicin-formaldehyde conjugate bound to AR-GFP but the resulting complex did not translocate to the nucleus. Binding was apparent from the drugs inhibition of Mibolerone-induced translocation. Direct observation of anthraquinone fluorescence of targeted drug in PC3 cells showed initial cytosolic localization, independent of AR expression, with predominant nuclear localization after sufficient time for release of drug from the targeting moiety. The results indicate that doxorubicin-formaldehyde conjugate bonded to cyanonilutamide via a cleavable linear tether enters PC3 cells, resides in cytosol, binds to the AR if present, and ultimately releases doxorubicin or a doxorubicin derivative to the nucleus.

Kinetic analysis of site-specific photoaptamer-protein cross-linking

ssDNA oligonucleotides containing bromodeoxyuridine, BrdU-photoaptamers, are rapidly emerging as specific protein capture reagents in protein microarray technologies. A mathematical model for the kinetic analysis of photoaptamer-protein photocross-linking reactions is presented. The model is based on specific aptamer/protein binding followed by laser excitation that can lead to either covalent cross-linking of the photoaptamer and protein in the complex or irreversible photodamage to the aptamer. Two distinct kinetic regimes, (1) frozen and (2) rapid equilibrium, are developed analytically to model binding kinetics between laser pulses. The models are used to characterize the photocross-linking between three photoaptamers and their cognate protein targets; photoaptamers 0650 and 0615 cross-link human basic fibroblast growth factor and 0518 cross-links HIV MN envelope glycoprotein. Data for cross-linking reaction yields as a function of both laser energy dose and target protein concentration are analyzed for affinity constants and cross-link reaction rates. The binding dissociation constants derived from the cross-linking data are in good accord with independent measurements; the rapid equilibrium model appears to produce results more consistent with the experimental observations, although there is significant overlap between the two models for most conditions explored here. The rate of photodamage for 0615 and 0518 is 3.5 and 2.5 times that of the specific cross-link, giving low maximum reaction yields of approximately 20% and approximately 30%, whereas 0650 cross-links with a rate over five times higher than its photodamage rate and has a maximum reaction yield exceeding 80%. Quantum yields for the three systems are estimated from the data; photoaptamer 0650 has a reasonably high quantum yield of approximately 0.2 for protein cross-linking, while 0518 and 0615 have quantum yields of 0.07 and 0.02. The work presented here provides a useful set of metrics that allow for refinement of photoaptamer properties.

Design, Synthesis, and Biological Evaluation of Doxorubicin−Formaldehyde Conjugates Targeted to Breast Cancer Cells

The anthracycline antitumor drug doxorubicin (DOX) has been utilized for decades as a broad-spectrum chemotherapeutic. Recent literature evidence documents the role of formaldehyde in the cytotoxic mechanism, and anthracycline-formaldehyde conjugates possess substantially enhanced activity in vitro and in vivo. Targeting a doxorubicin-formaldehyde conjugate specifically to cancer cells may provide a more efficacious chemotherapeutic. The design and 11-step synthesis of doxorubicin-formaldehyde conjugates targeted to the estrogen receptor, which is commonly overexpressed in breast cancer cells, are reported. The formaldehyde is incorporated in a masked form as an N-Mannich linkage between doxorubicin and salicylamide. The salicylamide triggering molecule, previously developed to release the doxorubicin-formaldehyde active metabolite, is tethered via derivatized ethylene glycols to an E and Z mixture of 4-hydroxytamoxifen. The targeting group, E/Z-4-hydroxytamoxifen, was selected for its ability to tightly bind the estrogen receptor and antiestrogen binding sites. The targeted doxorubicin-formaldehyde conjugates' estrogen receptor binding and in vitro growth inhibition were evaluated as a function of tether length. The lead compound, DOX-TEG-TAM, bearing a triethylene glycol tether, binds the estrogen receptor with a binding affinity of 2.5% relative to E/Z-4-hydroxytamoxifen and inhibits the growth of four breast cancer cell lines with 4-fold up to 140-fold enhanced activity relative to doxorubicin.

Rational Design and Synthesis of Androgen Receptor-Targeted Nonsteroidal Anti-Androgen Ligands for the Tumor-Specific Delivery of a Doxorubicin−Formaldehyde Conjugate

The synthesis and preliminary evaluation of a doxorubicin-formaldehyde conjugate tethered to the nonsteroidal antiandrogen, cyanonilutamide (RU 56279), for the treatment of prostate cancer are reported. The relative ability of the targeting group to bind to the human androgen receptor was studied as a function of tether. The tether served to attach the antiandrogen to the doxorubicin-formaldehyde conjugate via an N-Mannich base of a salicylamide derivative. The salicylamide was selected to serve as a trigger release mechanism to separate the doxorubicin-formaldehyde conjugate from the targeting group after it has bound to the androgen receptor. The remaining part of the tether consisted of a linear group that spanned from the 5-position of the salicylamide to the 3'-position of cyanonilutamide. The structures explored for the linear region of the tether were derivatives of di(ethylene glycol), tri(ethylene glycol), N,N'-disubstituted-piperazine, and 2-butyne-1,4-diol. Relative binding affinity of the tethers bound to the targeting group for human androgen receptor were measured using a (3)H-Mibolerone competition assay and varied from 18% of nilutamide binding for the butynediol-based linear region to less than 1% for one of the piperazine derivatives. The complete targeted drug with the butynediol-based linear region has a relative binding affinity of 10%. This relative binding affinity is encouraging in light of the cocrystal structure of human androgen receptor ligand binding domain bound to the steroid Metribolone which predicts very limited space for a tether connecting the antiandrogen on the inside to the cytotoxin on the outside.

Sensitivity and specificity of photoaptamer probes

The potential of photoaptamers as proteomic probes was investigated. Photoaptamers are defined as aptamers that bear photocross-linking functionality, in this report, 5-bromo-2'-deoxyuridine. A key question regarding the use of photoaptamer probes is the specificity of the cross-linking reaction. The specificity of three photoaptamers was explored by comparing their reactions with target proteins and non-target proteins. The range of target/non-target specificity varies from 100- to >10(6)-fold with most values >10(4)-fold. The contributions of the initial binding step and the photocross-linking step were evaluated for each reaction. Photocross-linking never degraded specificity and significantly increased aptamer specificity in some cases. The application of photoaptamer technology to proteomics was investigated in microarray format. Immobilized anti-human immunodeficiency virus-gp120 aptamer was able to detect subnanomolar concentrations of target protein in 5% human serum. The levels of sensitivity and specificity displayed by photoaptamers, combined with other advantageous properties of aptamers, should facilitate development of protein chip technology.

Evaluation of the epidoxorubicin--formaldehyde conjugate, epidoxoform, in a mouse mammary carcinoma model

This study was conducted to evaluate the toxicity and efficacy of Epidoxoform, a prodrug of the active metabolite of epidoxorubicin, in a mouse model of mammary carcinoma. A dose escalation trial established a maximal tolerated dose of 20 mg/kg given intraperitoneally (i.p.) to 6-8 week old female C3 HeJ mice. Two days following injection of 10(6) Gollin-B mouse mammary tumor cells into the mammary fat pad, Epidoxoform was given and tumor growth compared to mice treated similarly with the maximum tolerated dose of epidoxorubicin and untreated controls. In all efficacy trials, a significant difference was found for tumor volume between Epidoxoform and epidoxorubicin treated mice and controls. In mice treated with a two-dose regimen, significantly increased efficacy was also found between Epidoxoform compared to epidoxorubicin. Epidoxoform appears to be efficacious in this model of mammary carcinoma, with improved efficacy over the parent compound epidoxorubicin. Further evaluation of this analogue appears warranted.

Chemical ionization mass spectrometric determination of acrolein in human breast cancer cells

A selected ion flow tube-chemical ionization mass spectrometric method is presented for the first determination of acrolein metabolically produced in biological tissues. Acrolein in aqueous samples (2.5 ml) is preconcentrated by distillation and directly analyzed using gas-phase proton transfer from H3O+. This method provides sensitive detection of acrolein with the method detection limit of 15 nM at the 99% confidence level. Detection is linear up to the highest concentration studied (13.5 microM, R2 = 0.998). Acrolein levels are determined in doxorubicin-sensitive (MCF-7) and doxorubicin-resistant (MCF-7/Adr) human breast cancer cells in vitro. The intracellular acrolein concentrations differ insignificantly: 0.61 microM for sensitive cells and 0.54 microM for resistant cells. Treatment with a physiological concentration of doxorubicin (0.5 microM) for 24 h at 37 degrees C increased acrolein levels by factors of 2.6 and 1.9 for MCF-7 and MCF-7/Adr cells, respectively. The differential enhancement observed is consistent with the lower levels of enzymes that neutralize oxidative stress in sensitive MCF-7 cells and overexpression of an active drug efflux pump P-170 glycoprotein in resistant MCF-7/Adr cells.