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Polyesters from Reaction of 3,5-Pyridinedicarboxylic Acid and Group V-Containing Dihalides and Their Preliminary and Comparative Ability to Inhibit Cancer Cell Growth


Charles E. Carraher, Jr.1*, Michael R. Roner2, Kendra Black1, Jessica Frank1, Alisa Moric-Johnson2, Lindsey Miller2


1. Florida Atlantic University, Department of Chemistry and Biochemistry, Boca Raton, FL 33431

2. University of Texas Arlington, Department of Biology, Arlington, TX 76010


ABSTRACT

Group VA-containing polyesters are formed from the interfacial polymerization of the Group VA triphenylmetal dihalides with the salt of 3,5-pyridenedicarboxylic acid.  Yield and molecular weight decrease as the size of the metal atom increases.  Infrared spectroscopy, IR, shows formation of the M-O linkage with the geometry about the metal atom being a combination bridged and non-bridged.  MALDI MS shows the presence of metal-containing ion fragment clusters of 5 to 7 repeat polymer units.

Isotopic abundance for the antimony products is consistent with the presence of one, two, and three metal atoms in the particular ion fragment cluster. The polymers exhibit good inhibition of all of the tested cancer cell lines including two pancreatic cancer cell lines that represent about 90% of human pancreatic cancers.  In comparison to other metal-containing polymers derived from 3,5-pyridenedicarboxylic acid the Group VA polymers have EC50 values similar to organotin polymers and lower than for the Group IVB metallocene polymers.  Conversely, the metallocene polymers exhibit higher CI50 values.

Keywords: Group VA polymers, 3,5-pyridendicarboxylic acid, metal-containing polymers, interfacial polycondensation,  pancreatic cancer, breast cancer

INTRODUCTION

Our group has focused on the synthesis of a variety of metal-containing agents for different reasons including most recently combating unwanted microbes.  This effort has been reviewed for tin [1,2], platinum [3], Group IVB metallocenes [4]  and Group VA metals [5].  Recently we began synthesis of Group VA-containing polymers because some of these showed good inhibition of pancreatic cancer cell lines [6].

The biological activity of organoarsenic, organoantimony and organobismuth compounds is well established [6-10].  The topic of Group VA polymers has been reviewed [4,8,9].  A number of different condensation polymers have been synthesized from the reaction of simple organometallic dihalides or dinitrates with typical Lewis bases as diamines, salts of dicarboxylic acids, and diols  [10-16].

One driving force for our specific syntheses is to couple Lewis acids, generally the metal-containing moiety, that shows biological activity with Lewis bases, that also exhibit biological activity, hoping that the combination gives polymers with enhanced ability to inhibit unwanted microbes.

3,5-Pyridinedicarboxylic acid, PDA, also known as 5-carboxynicotinic acid, dinicotinic acid, and 3,5-pyridinedicarboxate (CAS 499-81-0) is a competitive inhibitor of bovine liver glutamate dehydrogenase [17,18].  Thus, it fits the approach of employing biologically active Lewis bases coupled with biologically active metal-containing Lewis acids.

There are a number of reports describing the synthesis of polymers from 3,5-pyridinedicarboxylic acid, PDA, often employing the diester of PDA rather than PDA itself [19-24].  For example, Ogata described the synthesis of various polyamides and polyesters from the reaction of the diesters with various diamines and diacid chlorides [19]. Marvel and Vogel reported the formation of polybenzimidazoles through reaction of the phenyl ester [20].

Similarly, a number of metal-containing condensation products have been synthesized including organotin products [25-27].  Coordination polymers have also been formed from PDA with terbium salts [28,29], organotin chlorides and oxides [30] and other metal ions [31-39].

We recently reported the synthesis of organotin and metallocene polymers derived from the reaction of organotin dihalides (Figure 1, left) [40] and metallocene dihalides (Figure 1, right) with 3,5-pyridinedicarboxylic acid (Figure 1, left) [41]. Here we report the synthesis of the analogous Group VA polyesters (Figure 2).

Figure 1: Repeat unit for the products of 3,5-pyridinedicarboxylic acid and organotin dihalides (left) and titanocene dichloride (right) where R represents simple chain extension

 

Figure 2:  Reaction between the salt of 3,5-pyridinedicarboxylic acid and triphenylantimony dichloride were R represents simple chain extension

 

EXPERIMENTAL

Synthesis

Reactions were carried out using the interfacial polycondensation technique.  An aqueous solution (10.0 ml) containing PDA, (0.00200 mol) and sodium hydroxide ( 0.0040  mol) was transferred to a one quart Kimax emulsifying jar fitted on top of a Waring Blender (model 1120; no load speed of about 18,000 rpm; reactions were carried out at about 25 oC).

Stirring was begun and a chloroform (for the triphenylantimony and arsenic solutions and carbon tetrachloride for the triphenylbismuth solution) solution (10.0 ml) containing the Lewis acid dihalide (0.00200 mol) was rapidly added through a hole in the jar lid using a powder funnel.  The resulting solution was blended for 5 seconds.  The precipitate was recovered using vacuum filtration and washed several times with deionized water and chloroform (or carbon tetrachloride) removing unreacted materials and unwanted by-products.  The solid was washed onto a glass Petri dish and allowed to dry at room temperature.

Triphenylarsenic dibromide (3313-89-1) was synthesized as described by Brickleband and co-workers [42]. Triphenylbismuch dichloride (594-30-9) and 3,5-pyridinedicarboxylic acid (499-81-0)  were purchased from Aldrich Chemical Co., Milwaukee, WS; triphenylantimony dichloride (594-31-0) dichloride was purchased from Strem Chemical Co., Newburyport, MA.

Physical Characterization

Light scattering photometry, employing a Brice-Phoenix Universal Light Scattering Photometer Model 4000, was used to obtain polymer molecular weight from DMSO solutions.  Infrared spectra were obtained employing attenuated total reflectance infrared spectroscopy utilizing a Thermo Scientific Nicolet iS5 FTIR equipped with an id5 ATR attachment. 1H NMR spectra were obtained in d-6 DMSO employing Varian Inova 400 MHz amd Varian 500 MHz spectrometers.

High resolution electron impact positive ion matrix assisted laser desorption ionization time of flight, HR MALDI-TOF, mass spectrometry was carried out employing a Voyager-DE STR BioSpectrometer, Applied Biosystems, Foster City, CA. The standard settings were used with a linear mode of operation and an accelerating voltage of 25,000 volts; grid voltage 90% and an acquisition mass range of 500 to 2,500 Da.  A graphite matrix was employed.  Graphite from a number 2 pencil was marked on the sample holder and sample placed onto the graphite mark.

Cell Testing

The toxicity of each test compound was evaluated for a group of cell lines.    Cells were seeded into a 96-well culture plate at a density of 20,000 cells per well in 100 µL of culture medium. Following a 24 h incubation period, the test compounds were added at concentrations ranging from 0.0032 to 32,000 ng/ml and allowed to incubate at 37°C with 5% CO2 for 72 h. Following incubation, Cell Titer-Blue reagent (Promega Corporation) was added (20ul/well) and incubated for 2 h. Fluorescence was determined at 530/590 nm and converted to % cell viability versus control cells.

All cytotoxicity values are calculated against a base-line value for each line that was generated from “mock-treatment” of the normal and tumor cell lines with media supplemented with all diluents used to prepare the chemotherapeutic compounds.  For example, if the compounds were dissolved in DMSO and serial dilutions prepared in MEM to treat the cells, then the mock-treated cells were “treated” with the same serial dilutions of DMSO without added chemotherapeutic compound.

This was done to ensure that any cytotoxicity observed was due to the activity of the compound and not the diluents.  For the studies reported here, the mock-treatment never resulted in a loss of cell viability of more than one percent, demonstrating that the activity observed was not due to cytotoxicity of any of the diluents used, but was due to activity of the tested compounds. The inhibition curve is sigmoid and the EC50 determined at the midpoint of the curve.  Once inhibition begins the concentration difference between the initial inhibition and final total inhibition is small with the region between initial inhibitions to final total inhibition essentially linear.

RESULTS AND DISCUSSION

Yield and Chain Length

Table 1 presents the yield and average chain length (degree of polymerization, DP-simply number of repeat units) for products formed from reaction of the salts of PDA and Group VA triphenylmetal dihalides.

Table 1: Product yield, molecular weight and chain length as a function of Lewis Base

Lewis Base %  Initial Yield Total Yield Mol. Wt. DP
Ph3AsBr2 13 21 9.0 x 105 1900
Ph3SbCl2 6 20 2.2 x 104 43
Ph3BiCl2 4 16 1.5 x 104 25

 

Reaction is rapid giving product within five seconds stirring.  The rapidity of the polymerization is a consequence of the low activation energy associated with the reaction of the metal-containing halide and Lewis base [1,2,4]. Yield is decent ranging from 16-21%.  As in other polymerizations employing Group VA triphenylmetal halides, additional product precipitates from the reaction system after the initial product is formed [5,6,12-16]. Column 3, Table 1, contains the total yield, including that initially precipitated, column 2.

The arsenic products are a high polymer, but the other two, while polymers, are moderate chain length.  There is a slight trend with respect to yield, chain length and the metal atom such that both yield and chain length decrease as the size increases and electronegative decreases.  The arsenic product is a brown color because of the presence of the As-Ph brown color site.  The bismuth and antimony products are white because of the absence of a color site in the camphoric acid and triphenylantimony and triphenylbismuth moieties.

Because the diacid form of 3,5-pyridinedicarboxylic acid is not a sufficiently strong nucleophile, base is added converting 3,5-pyridinedicarboxylic acid to its salt which is a reasonably strong nucleophile allowing formation of the desired ester linkage.

Infrared Vibration Spectroscopy

Infrared spectral analysis was conducted on the products and reactants and assignments are based on prior work by us and others [5,6,12-16,40,41,43-46].  Results for the products appear in Tables 2 and 3.  The presence of C-H stretching bands appearing in the aromatic region about 3000 cm-1 (all vibration bands are given in wave numbers, cm-1) characteristic of both camphoric acid and the triphenylmetal moieties (above 3000) are present consistent with the product containing moieties from both reactants.  New bands appear about 1260 (symmetrical stretch) and 740 (asymmetrical stretch) assigned to the formation of the M-O linkage.  Other important bands and assignments appear in Tables 2 and 3.

The esters can exist as bridging or distorted seven-bonded bridged structures (Figure 3, left) and non-bridging or triangular bipyramidal structures (Figure 3, right) with respect to the geometry about the metal atom. Infrared spectroscopy is the easiest way to determine the structure about the metal.  Bridging asymmetric carbonyl absorptions are found around 1550-1600 (all infrared bands are given in cm-1).   The bridging symmetric carbonyl band is found around 1410-1440.  Non-bridging asymmetric carbonyl bands are found about 1600-1700; and the corresponding symmetric carbonyl band found about 1330-1345. 

Table 3 contains bands associated with bridging and non-bridging for the polymers. Bands associated with both bridging and non-bridging are found so the product contains a mixture of bonding about the metal atom.  Bands associated with non-bridging increase as the metal atom increases in size while those associated with bridging increase as the metal atom size decreases.  The larger atom size requires that bridging occur over an energetically less favorable longer distance. 

Table 2: Selected infrared spectral band assignments for titanocene dichloride, PDA and the polymers derived from reaction of various metallocene dichlorides with PDA.

 

Assignment Ph3AsBr2 PDA Ph3As/PDA Ph3SbCl2 Ph3Sb/PDA Ph3Bi/PDA
C-H st (Ar) 3103 3090 3090,3052 3060 3092,3067 3091,3054
C=O St, non-bridg 1640 1638 1638 1635
Ring st 1565 1578 1575 1575
C=O St, bridging 1588 1597 1556
C=C st 1465 1465 1460 1460 1469
M-Ph 1438 1437 1440 1437 1437
C=C ip 1362 1366 1368 1368
OH ip wag 1337,1330
M-O asy st 1265 1263 1274
C-C 1152 1157 1152 1160 1160
M-Ph sym st 1050 1083 1048 1050 1050
CH ip wag 1015 1015 1021 1027
Ring Breathing 997 996 996 995 997 984
CH op wag 889 900 890 890 899
Skelatal-C Inversion 854 848 840 850 850
CH op bending 820 828 818 825
M-O sym st 737 732 735
Aryl op 690 682 690 687 694

Where op = out of plane; ip= in plane; sys=symmetrical; st=stretching; asy=asymmetric

 

Figure 3:  Geometrical arrangements about the metal atom, here the bismuth metal atom

Table 3:  Presence of bridging and non-bridging associated bands and location 

Organometal Moiety Asym bridging Sym bridging Asym Non- Bridging Sym Non- Bridging
Ph3As 1588(m) 1437 (l) 1638(m) 1335(s)
Ph3Sb 1597(s) 1436(s) 1647(m) 1333(l)
Ph3Bi 1556(s) 1436(s) 1633(l) 1334(l)

 Where l = large, m= moderate, s= small;

 

Matrix-Assisted Laser Desorption/Ionization Mass spectrometry

While Matrix-assisted laser desorption/ionization mass spectrometry, MALDI MS was developed for the analysis of non-volatile samples with special use in the identification of polymers its potential for the analysis of a wide range of materials has not been fulfilled.  This is largely due to the inability of many materials to be soluble in somewhat volatile liquids to a decent extent allowing good mixing of the polymer with matrix material. This has largely eliminated most synthetic high-polymers from being suitably analyzed employing MALDI MS.

For about a dozen years we and others have been employing MALDI MS for the identification of a number of non-volatile metal and non-metal containing polymers. This has been recently reviewed [47-50]. The technique employed by us is not straight forward MALDI MS but it is applicable to soluble and insoluble products so has wide potential for application.  The technique focuses on the fragments that are created in the MALDI MS process.

Recently graphite has been employed by us as the matrix material because it gives good results with few interfering ion fragments produced above 500 mass which is the typical lower mass range employed in our studies [51, 52].   Two general MALDI MS modes were employed. These are the reflective and linear mode. The reflective mode has a longer focal length than the linear mode.  Results for the reflective mode allow finer features, such as isotopic abundances, to be more accurately determined but generally results in the detection of lower masses.  By comparison, the linear mode has a shorter flight distance and results in the detection of higher masses. Following are results for the polymers.

The major ion fragments for the triphenylantimony product with 3,5-pyridinedicarboxylic acid, PDA, are given in Table 4 and shown in Figures 4 and 5.  Abbreviations are employed to describe tentative assignments.  These are PDA for 3,5-pyridinedicarboxylic acid minus two protons; Ph is the phenyl moiety; U is one repeat unit; 2U is two repeat units.  (Examples of repeat units are given in Figures 2.) Sodium is a common contaminant.  All masses are given in Daltons, Da.

 

Figure 4:  MALDI MS of the product of triphenylarsenic dibromide and PDA over the mass range of 500 to 1200 Da using the reflective mode

Figure 5:  MALDI MS of the product of triphenylarsenic dibromide and PDA over the mass range of 500 to 1200 Da using the linear mode

 

Table 4:  Major ion fragments for the product of triphenylarsenic dibromide and PDA

Mass,Da/Linear Mass,Da/Reflective (Tentative) Assignment Mass,Da/Linear Mass,Da/Reflective (Tentative)

Assignment

579 U+PDA-CO2,O 1319 3U-Ph,O
605 607 U+PDA-20 1366 3U-Ph+Na
639 U+PDA 1401 3U-O
645 645 U+PDA,Na-O 1582 3U+PDA
662 U+PDA,Na 1661 3U+ Ph2As,O
715 U+ Ph2As,O 1711 3U+ Ph3As
749 749 U+ Ph2As,CO2 1853 4U-O
781 U+ Ph3As 1959 4U+PDA-Ph,O
820 U+ Ph3As,CO2 2077 4U+PDA,Na
856 U+ Ph3As,2CO2 2180 4U+ Ph3As
883 886 U+ Ph3As,2CO2,Na 2225 4U+ Ph3As,2O
912 910 2U-2O 2353 5U
926 2U-O 2371 5U-O
946 2U 2502 5U+PDA-O
987 2U+CO2 2618 5U+ Ph2As,2O
1005 2U+CO2,Na 2738 5U+ Ph3As,CO2,O,Na
1117 2U+PDA 2891 6U+CO2,Na
1165 2U+ Ph2As 3041 6U+ Ph2As,O
1186 2U+ Ph2As,O 3268 7U-O
1215 2U+ Ph2As,CO2 3345 7U+CO2
1268 2U+ Ph3As,O

Ion fragments are found corresponding to up to seven units long.

Table 5 contains results for the product of triphenylantimony dichloride and PDA.  Ion fragments to over 6 units are found.

Table 5:  Major ion fragments for the product of triphenylantimony dichloride and PDA

Mass,Da/

Linear

Mass,Da/

Reflective

(Tentative) Assignment Mass,Da/

Linear

Mass,Da/

Reflective

(Tentative)

Assignment

526 U-O 1744 3U+PDA,Na
608 609 U+PDA-CO2,O 1894 3U+ Ph3Sb
635 U+PDA-CO2 2021 4U-CO2
791 794 U+Ph2Sb 2071 4U
857 856 U+ Ph3Sb 2232 4U+PDA
883 883 U+ Ph3Sb,O 2387 4U+ Ph2Sb,CO2
912 U+ Ph3Sb,CO2 2485 4U+ Ph3Sb,CO2.O
966 U+ Ph3Sb,2CO2 2635 5U+CO2
1032 1035 2U 2763 5U+PDA,Na-O
1415 2U+ Ph3Sb,2O 3054 6U-CO2
1498 3U+Na-Ph 3197 6U+PDA-Ph
1523 3U-CO 3345 6U+Sb,CO2
1598 3U+CO2

 

Antimony contains two isotopes allowing isotopic matches to be made. Table 6 contains such matches containing one, two, and three antimony atoms.  The structures given in the top line of each table correspond to the structure of the particular ion fragment cluster. The agreement to the expected listed as “Standard”, appearing in the two most left-hand columns, is reasonable consistent with the present of one, two, and three antimony atoms within these ion fragment clusters.

 

Table 6: Isotopic abundance matches for ion fragments derived from the product of triphenylantimony dichloride and PDA

 

Known for Sb U+PDA-CO2,O U+PDA-CO2
121 100 608 100 635 100
123 75 610 74 637 76

 

 

Known for 2Sb U+Ph2Sb U+Ph3Sb,O
242 67 792 68 881 69
244 100 794 100 883 100
246 37 796 34 885 38

 

Known for 3Sb 3U-CO 3U+CO2
363 45 1521 41 1596 46
365 100 1523 100 1598 100
367 75 1525 77 1600 74
369 19 1527 21 1602 18

Table 7 contains the major ion fragment found for the product of triphenylbismuth and PDA.

Table 7: Major ion fragment clusters for the product from triphenylbismuth dichloride and PDA

Mass,Da/

Linear

Mass,Da/

Reflective

(Tentative)

Assignment

Mass,Da/

Linear

Mass,Da/

Reflective

(Tentative)

Assignment

606 606 U 1574 2U+ Ph2Bi
643 645 U+CO2 1621 1625 2U+ Ph2Bi+CO2,O
745 U+PDA-CO2 1710 2U+ Ph2Bi+CO2,Na
885 881 U+Bi 1847 1850 3U+CO2
912 U+Bi,Na 2003 3U+PDA,Na
923 924 U+Bi,2O 2216 3U+ Ph2Bi+CO2
943 942 U+Bi,CO2 2240 3U+ Ph3Bi
1012 1010 U+Bi,2CO2,Na 2270 3U+ Ph3Bi,O
1071 U+ Ph2Bi,2CO2,Na 2324 3U+ Ph3Bi,CO2
1141 2U-Ph+O 2342 2340 3U+ Ph3Bi,CO2,O,Na
1160 2U-Ph+CO2 2619 4U+PDA,Na
1187 2U-Ph+CO2,Na 2740 4U+Bi,Na
1266 1267 2U+CO2 3140 5U+PDA-CO2
1306 2U+PDA-CO2,O 3164 5U+PDA-2O
1326 2U+PDA-CO2

 

Ion fragments to over five repeat units are found.

There is little loss of the phenyl unit on the metal.  As in other studies, this loss occurs at the site of bond scission [43-46,49].  The pyridine ring unit remains intact consistent with the mildness of MALDI MS.  As in other studies, bond scission occurs at the hetero-bond sites in the polymer chain.  Figure 6 shows these sites for the repeat unit from triphenylbismuth/PDA.

Figure 6: Locations of preferred chain bond scission

In summary, MALDI MS shows the presence of ion fragment clusters to 7 units of the arsenic product, 6 units for the antimony product, and 5 units for the bismuth.  This trend occurs because it represents the upper mass limit used for each product and not for some other reason.

Proton NMR spectroscopy

Proton NMR was carried out on the products and monomers. PDA shows two proton environments associated with the pyridine ring (Figure 7).  For PDA these bands appear for environment “a” at 7.4 and for “b” at 6.8 (all band locations are given in ppm). For the polymers these bands appear between 7.4-7.5 and 6.8 consistent with the presence of the PDA moiety. Because the protons in the 3,5-pyridinecarboxylic ring are isolated from the organometallic moiety, these bands appear little changed between the monomer and the various polymers.   Triphenylarsenic dibromide shows three bands all derived from the phenyl group (ortho-, meta-, para- hydrogen atoms) at 8.20, 7.70, and 7.60. The triphenylarsenic/PDA polymer shows bands from the triphenylarsenic moiety at 8.5, 7.7 and 7.6 and from the PA moiety at 7.4 and 6.7. Triphenylantimony dichloride shows three bands at 8.20, 7.70, and 7.60.  The corresponding polymer shows bands at 8.2, 7.7 and 7.6 from the triphenylantimony moiety and from the PDA ring protons at 7.4 and 6.8.  Thus, proton NMR spectroscopy is consistent with the presence of units derived from both reactants. Because of the poor solubility of the polymers, additional implications from the data are not confidently derived.

Figure 7:  3,5-Pyridinedicarboxylic acid with assigned protons locations

Tumor Analysis

The battery of test cancer cell lines used in this study is given in Table 8.

Table 8: Cell lines employed in the current study

 

Strain # NCI Desig. Species Tumor Origin Histological Type
3465 PC-3 Human Prostate Carcinoma
7233 MDA MB-231 Human Pleural effusion breast Adenocarcinoma
1507 HT-29 Human Recto-sigmoid colon Adenocarcinoma
7259 MCF-7 Human Pleural effusion-breast Adenocarcinoma
ATCC CCL-75 WI-38 Human Normal embryonic lung Fibroblast
CRL-1658 NIH/3T3 Mouse Embyro-continuous cell line of highly contact-inhibited cells Fibroblast
AsPC-1 Human Pancreatic cells Adenocarcinoma
PANC-1 Human Epithelioid pancreatic cells Carcinoma

 

In other studies we found that the polymer drugs are cytotoxic and cell death is by necrosis [1.2.53]. We have recently found that the anticancer activity is brought about by the intact polymer and not through polymer degradation [1-3].  This is consistent with studies that show the polymers are stable in DMSO with half-chain lives, the time for the polymer chain length to halve, generally in excess of 30 weeks [1-3,53]. Further, since the products are formed using the condensation process and are collected as a precipitate, the molecular weight distribution is believed to be relatively narrow [54-58].  Also, it is well known that most organometallic compounds associate with polar solvents such as DMSO and that the biological results are somewhat influenced by the presence of the DMSO [1,3,53,59-61].  For polymers similar to those described in the present study, this influence is found to be small, generally less than 20% [1-3].

While different measures are employed in the evaluation of a materials ability to inhibit cell growth, the two most widely used are employed in the present study. The first is the concentration dose needed to reduce growth of a particular cell line.  Several names are associated with this concentration.  The term effective concentration, EC, will be employed here.  The concentration of a drug, antibody, or toxicant that induces a response halfway between the baseline and maximum after a specified exposure time is referred to as the 50% response concentration and is given the symbol EC50.

Table 9 contains the EC50 values for the monomers and polymers. The cells represent a broad range of cancers. For comparison, values for cisplatin are also given.  Cisplatin is among the most widely employed anticancer drugs. It is considered highly toxic and this is shown by the low EC50 values found towards the standard cell lines, WI-38 and NIH 3T3 cells.   PDA shows no inhibition to the limits tested for most of the cancer cell lines and in the single case where it does, inhibition is only mild compared to the polymers.  Further, the metal-containing monomers also exhibit mild inhibition of the cancer cell lines but again much less than the metal-containing polymers.  Thus, neither monomer exhibits high inhibition of any of the cancer cell lines and it is the polymeric combination of the two reactants that is responsible for the observed ability to inhibit cancer cell line growth.

Much of our recent effort has been on discovering compounds that inhibit pancreatic cancer because pancreatic cancer does not have a generally accepted “cure” [1,2,62-65]. Thus, the set includes two widely employed pancreatic cell lines. These are the AsPC-1 cell line which is an adenocarcinoma pancreatic cell line representing the most often observed pancreatic cancer cell line found in humans (about 80%) and the PANC-1 cancer cell line which is an epithelioid carcinoma pancreatic cell line representing the second most frequently observed pancreatic cancer cell line found in humans (about 10%).  All three of the polymers show good inhibition of both pancreatic cancer cell lines.  The inhibition of the pancreatic cancer cell lines is similar for both the ASPC-1 and PANC-1 cells indicating that inhibition by the polymers may be general for the other pancreatic cancers.

The two breast cancer cell lines represent a matched pair.  The MDA-MB-231 (strain number 7233) cells are estrogen-independent, estrogen receptor negative while the MCF-7 (strain line 7259) cells are estrogen receptor (ER) positive.  In some studies involving organotin polymers there was a marked difference between the ability to inhibit the two cell lines dependent on polymer structure [2.3.66-68]. In the current study there is little difference in the ability to inhibit the two cell lines by the polymers with the polymers inhibiting both breast cancer cell lines with about the same EC50.  The polymers also exhibit good inhibition of the prostrate (PC-3) and colon (HT-29) cancer cell lines.

All three polymers show good ability to inhibit all of the tested cell lines, cancer and standard.  Further, inhibition is similar regardless of the nature of the metal.  It is the combination of the PDA with the metal-containing moiety that allows the polymer to effectively inhibit cell growth rather than the particular metal-containing moiety.  In summary, based on EC50 values, the polymers show good ability to inhibit a variety of cancer cell lines.

Table 9:  EC50 Concentrations (micrograms/mL) for the tested compounds. Values given in ( ) are Standard Deviations for each set of measurements

 

Sample 3465/PC-3 7233/MDA 1507/HT-29 7259/MCF-7
Ph3AsBr2 20.0(1.7) 21.4(1.0) 16.5(1.2) 24.6(2.1)
Ph3As/PDA 0.55(.6) 0.58(.7) 0.59(.7) 0.55(.7)
Ph3SbCl2 38(4) 12.4(1.1) 33.(3.1) 135(11)
Ph3Sb/PDA 0.53(.6) 0.56(.7) 0.55(.7) 0.58(.7)
Ph3BiCl2 2.4(.16) 1.4(.2) 2.2(.16) 1.6(.21)
Ph3Bi/PDA 0.55(.6) 0.59(0.7) 0.58(.7) 0.60(.5)
PDA >32000 >32000 >32000 >32000
Cisplatin 0.0044(.004) 0.0029(.002) 0.0041(.003) 0.0057(.003)

The second widely used measure of cancer inhibition ability involves comparing the amount of drug needed to inhibit the cell growth of a standard cell line compared to the amount to inhibit the cell growth of a particular cancer cell line.  Again, a variety of symbols are employed to describe similar calculations.  Here, the term chemotherapeutic index, CI, will be used so that the CI50 is then the ratio of the EC50 for the standard cell lines NIH 3T3 or WI-38 cells divided by the EC50 for the particular test cell. Results are given in Table 10.

Two cell lines are typically employed as standards in the evaluation of the effectiveness of compounds to arrest the growth of tumor cell lines.  These two cell lines are the NIH 3T3 and WI-38 cell lines.  The current study has two parts.   The first is evaluation of the CI50 values determined using the two different standard cells. NIH 3T3 cells are mouse embryo fibroblast cells.  They are part of a group of cell lines are referred to as partially transformed cells in that they are immortal unlike normal cells.  They retain other characteristics of normal cells such as being contact-inhibited. Relative to most normal cells they are robust and easily maintained. WI-38 cells are normal embryonic human lung fibroblast cells. They have a finite life time of about 50 replications.  Compared to NIH 3T3 cells, they are more fragile and difficult to maintain for long periods of time.  Thus, NIH 3T3 cells are often favored because of ease of handling aided by an infinite life span. But, when there is a difference, CI50 values determined using the WI-38 cell are favored [1-3].  For the current study, the CI50 values using either standard cell line are similar so that either could be used in this evaluation.

The second part of the study is to use the CI50 values to ascertain if preferential inhibition by the test compounds is present.  The CI50 values derived from values given in Table 10 are derived from values given in Table 9.  It is preferential to find large values meaning that the tendency to inhibit the cancer cells is preferential compared to the standard cell lines.  In general, CI50 values of two and greater are considered significant.  Focusing on the three polymer samples, there are no values greater than two so that there is not a major preferential for the test compounds to inhibit cancer cell growth.

Table 10:  CI50 results for values calculated from data given in Table 9  

Sample EC50 WI-38/

EC50AsPC-1

EC50 3T3/

EC50AsPC-1

EC50 WI-38/

EC50PANC-1

EC50 3T3/

EC50PANC-1

Ph3As/ PDA 1.1 1.0 1.1 1.1
Ph3Sb/ PDA 1.2 1.1 1.1 1.0
Ph3Bi/ PDA 1.2 1.1 1.0 0.97

 

Sample EC50 WI-38/

EC507233

EC50 3T3/

EC507233

EC50 WI-38/

EC507259

EC50 3T3/

EC507259

Ph3As/PDA 1.1 1.0 1.1 1.1
Ph3Sb/ PDA 1.1 1.1 1.1 1.0
Ph3Bi/ PDA 1.0 0.95 1.0 0.93

 

Sample EC50 WI-38/

EC501507

EC50 3T3/

EC501507

EC50 WI-38/

EC503465

EC50 3T3/

EC503465

Ph3As/ PDA 1.1

 

1.0 1.1 1.1
Ph3Sb/ PDA 1.1 1.1 1.2 1.1
Ph3Bi/ PDA 1.0 0.97 1.1 1.0

 

Low EC50 values show that the polymer inhibits cancer growth at low concentrations.  By comparison, high CI50 values are consistent with the polymer preferentially inhibiting cancer cell growth.  Researchers disagree as to which values is most important in indicating the drug effect in animals, the EC50 or CI50 [2,40,41,65-67].  Here, the EC50 values for the Group VA polymers show good inhibition of the cancer cell lines but the low CI50 values are consistent with little preferential inhibition of the cancer cell lines by the polymers.

COMPARISON OF CELL RESUTLS

We have synthesized PDA-containing polymers containing organotin [40], metallocene groups [41] and in this report Group VA units.  Following is a brief study of the ability of these polymers to inhibit cancer cell lines allowing a comparison of the effect of the particular metal atom on the ability to inhibit cancer cell growth.  Table 11 contains EC50 values for the various polymers. Values for the current new polymers are added for easy referral.

Table 11:  EC50 values in microgram/mL as the metal-containing moiety is varied

 

WI-38 PAN-1 AsPC-1 PC-3 MDA HT-29 MCF-7
Cp2Ti/PDA 18 2.1 4.4 5.1 5.7 2.3 4.4
Cp2Zr/PDA 20 1.1 5.5 17 19 14 18
Cp2Hf/PDA 20 4.6 3.7 1.1 4.1 4.7 2.7
Cp2V/PDA 20 2.2 7.5 3.1 6.6 4.0 2.8
Me2Sn/PDA 0.71 0.61 0.60 0.62 0.62 0.62 0.61
Et2Sn/PDA 0.72 0.71 0.69 0.67 0.60 0.64 0.63
Bu2Sn/PDA 0.63 0.69 0.66 0.70 0.63 0.67 0.67
Oc2Sn/PDA 0.68 0.62 0.63 0.60 0.71 0.63 0.62
Ph2Sn/PDA 0.62 0.64 0.67 0.67 0.63 0.68 0.66
Ph3As/PDA 0.62 0.55 0.58 0.55 0.58 0.59 0.55
Ph3Sb/PDA 0.61 0,57 0.53 0.53 0.56 0.55 0.58
Ph3Bi/PDA 0.60 0.58 0.52 0.59 0.59 0.58 0.60

 

With respect to EC50 values, those polymers where the metal is organotin and Group V have the lowest values showing the greatest ability to inhibit cancer growth. The EC50 values are similar for all of the organotin and Group V metal derived polymers and cancer cell lines.  By comparison, the results for the metallocenes are varied but in all cases the EC50 values are greater than for the organotin and Group V derived polymers.  For the metallocenes the EC50 values vary with the cancer cell line and metal though the values are generally within a decade of one another. Further, the Group IVB metallocene polymers are the least toxic towards the WI-38 human non-cancer cell line.

 

Table 12 contains the CI50 values calculated from the data given in Table 11.

Table 12:  CI50 values calculated from data given in Table 11

EC50WI-38/ EC50PANC-1 EC50WI-38/ EC50AsPC-1 EC50WI-38/ EC50PC-3 EC50WI-38/ EC50MDA EC50WI-38/ EC50HT-29 EC50WI-38/ EC50MCF-7
Cp2Ti/PDA 8.6 4.1 3.5 3.2 7.8 4.1
Cp2Zr/PDA 1.8 3.6 1.2 1.1 1.4 1.2
Cp2Hf/PDA 4.4 5.4 18 4.9 4.3 7.4
Cp2V/PDA 9.1 2.7 6.5 3.0 5.0 7.1
Me2Sn/PDA 1.1 1.2 1.1 1.1 1.2 1.3
Et2Sn/PDA 1.0 0.80 1.2 1.2 1.1 1.1
Bu2Sn/PDA 0.91 0.95 0.90 1.0 0.94 0.94
Oc2Sn/PDA 1.1 1.1 1.1 0.96 1.1 1.1
Ph2Sn/PDA 0.97 0.93 0.93 0.98 0.91 0.94
Ph3As/PDA 1.1 1.1 1.1 1.1 1.0 1.1
Ph3Sb/PDA 1.1 1.1 1.1 1.1 1.1 1.2
Ph3Bi/PDA 1.0 0.95 1.0 1.0 0.97 1.1

 

CI50 values of two and larger are generally considered significant [1-3].  Only the metallocene products exhibit such values with the highest values found for the titanocene, hafnocene, and vanadocene polymers showing good CI50 values for all of the cancer cell lines.

Thus, while the metallocene polymers are the least toxic towards the cancer cell lines as measured through EC50 values they show the highest CI50 values.  Further, for these polymers, the organotin and Group V products exhibit similar good toxicity towards the cancer cell lines as measured by EC50 values but the CI50 values exhibit little differentiation with respect to toxicity between the standard WI-38 and cancer cell lines.

Additionally, all of the products described here, except for the triphenylarsenic dibromide, are commercially available. Also, the interfacial polycondensation system employed in the synthesis of the polymers is commercially employed to synthesize aramid fibers and polycarbonates. Thus, scale-up can be readily accomplished.   

 CONCLUSIONS

Group VA-containing polyesters have been synthesized from the interfacial polymerization of 3,5-pyridinedicarboxylic acid with Group VA triphenyl organometallic dihalides with yield and chain decreasing as the size of the metal increases. With the exception of the triphenylarsenic dibromide, all of the reactants are commercially available.  Further, the interfacial polycondensation system is industrially employed in the synthesis of aramides and polycarbonates so that the polymers can be readily synthesized in milligram to ton quantities. IR shows new bands assigned to the formation of the M-O linkage and is consistent with the geometry about the metal atom being of a combination of bridging and non-bridging.  MALDI MS shows the formation of ion fragment clusters of 5 to 7 repeat units with breakage occurring at the heteroatom sites.  The polymers show good inhibition of all of the tested cancer cell lines including two pancreatic cancer cell lines.  A comparison of polymers derived from various metal-containing reactants and 3,5-pyridinedicarboxylic acid show similar EC50 values when the metal is Group VA and tin, but the CI50 values are more favorable for the metallocene polymers.

REFERENCES

  • Carraher C: Macromolecules Containing Metal and Metal-Like Elements, Vol. 4. Group IVA Polymer. Wiley, Hobokin,
  • Carraher C, Roner MR: Organotin polymers as anticancer and antiviral agents. Organomet. Chem. 2014; 751:67-82.
  • Roner MR, Carraher C, Shahi K, Barot G: Antiviral Activity of Metal-Containing Polymers-Organotin and Cisplatin-Like Polymers. Materials 2011; 4:991-1012.
  • Carraher : Condensation metallocene polymers. Inorg. Organometal. Polyms. 2005; 15:121-145.
  • Carraher C: Antimony-containing polymers. Polym. Mater. 2008; 25:35-50.
  • Carraher C, Truong N, Roner MR, Moric A, Trang N: Synthesis of organoarsenic, organoantimony, and organobismuth poly(ether esters) from reaction with glycyrrhetinic acid and their preliminary activity against pancreatic cancer cell lines. JCAMS 2013; 1:134-150.
  • Naka K, Chujo Y: Organic-inorganic hybrid polymers employing characteristics of hetero atoms. Kanaku to Kogyo 2007; 60:520-523.
  • Karak K, Maiti S, Das S, Dey SH: Antimony polymers part 5. Biological activity. Polym. Mater. 2003; 20:237-242.
  • Karak N, Maiti S: Antimony containing polymers. Polym. Mater. 1996; 13:179-190.
  • Karak N, Maiti S: Antimony polymers. part 2. Physical, chemical, and thermal properties. Angew. Chemie 1999; 265:5-12.
  • Carraher C: Biological activities and medical applications of metal-containing macromolecules. in Bioactive Polymeric Systems.  Plenum, NY, 1985; 651-674.
  • Sabir T, Carraher C: Synthesis of triphenylantimony and triphenylbismuth-containing polyether amines containing acyclovir. Polym. Mater. 2006; 4:403-413.
  • Carraher C, Hedlund L: Synthesis and characterization of antimony (V) polyoximes, Macromol. Sci.-Chem. 1980; A14:713-728.
  • Carraher C, Venable W, Blaxall HS, Sheats JE: Synthesis and characterization of antimony (V)-polycobalticinium exters. Macromol. Sci.-Chem. 1980; A14:571-579.
  • Carraher C, Blaxall HS: Synthesis and solution characterization of antimony polyesters. Angew. Chemie 1979; 83:37-45.
  • Carraher C, Naas M, Giron DJ, Cerutis DR: Structural and biological characterization of antimony V polyamines. Macromol. Sci.-Chem. 1983; A19:1101-1120.
  • Lee W, Elliott J, Brownsey R: Inhibition of acetyl-CoA carboxylase isoforms by pyridoxal phosphate. Bio. Chem. 2005; 280:41835-41843.
  • McCann K, Laane J: Ramon and infrared spectra and theoretical calculations of dipicolinic acid, dinicotinic acid, and their dianions. Mol. Struc. 2008; 890:346-358.
  • Ogata N: Synthesis of polyamides and polyesters having various functional groups. Macromol. Sci. Chem. 1979; A13:477-501.
  • Marvel C, Vogel H.   US Pat. 3174947. 1965.
  • Hoff H, Krieger A: Polyamides from heterocyclic dicarboxylic acids.  Makromolecular Chemie 1961; 47:93-113.
  • Shizunobu H, Yasuhiko N: Synthesis of polyesters containing pyridine rings in the main chains. Kobunshi Kagaku 1967; 24:215-223.
  • Braz GI, Kardash IE, Yakubovick VS, Myasnikova GV, Ardashnikov A, Oleinik A, Pravednikov A, Yakubovick A: Polybenzosazoles, their synthesis and thermal degradation. Vysoko moleculyarnye Soedineniya 1966; 8:272-277.
  • Hergenrother P, Wrasidlo W, Levine H: Polybenzothiazoles. I. Synthesis and preliminary stability evaluation. Polym. Sci, Part A. 1965; 3:1665-1674.
  • Szorcsik A, Nagy L, Scopelliti M, Deak A, Pellerito L, Galbacs G, Hered M: Preparation and structural characterization of [Ph3Sn(IV)]+ complexes with pyridine-carboxylic acids or hydroxypyridine, -pyrimidine and –quinolone. Organometallic Chem. 2006; 691:1622-1630.
  • Szorcsik A, Nagy L, Deak A, Scopelliti M, Fekete Z, Csaszar A, Pellerito C, Pellerito L: Preparation and structural studies on the Bu2Sn(IV) complexes with aromatic mono- and dicarboxylic acids containing hetero {N} donor atmo. Organometallic Chem. 2004; 689:2762-2769.
  • Ma C, Li J, Zhang R: Synthesis and crystal structures of polymeric ionic triorganotin esters of 3,5-pyridinedicarboxylic acid and 5-nitroisophthalic acid. Coordination Chem. 2006; 59:1891-1904.
  • Qisong H, Benfeng H, Qiuyan L, Taiqi L: Synthesis, crystal structure and fluorescence property of terbium coordination polymers with pyridine-3,5-dicarboxylic acid. Materials Science Forum 2010; 663:72-75.
  • Shi, F. Liang. Synthesis, crystal structure and characterization of a novel terbium fluorescent coordination polymer.  Xiyou Jinshu Cailiao Yu Gongcheng, 2010; 39, 1202-1205.
  • Chandrasekhar V, Thirumoorthi J: Self-assembly of triorganotin (V) moieties with 1,2,4,5,-benzenetetracarboxylic acid: synthesis, characterizations andinfluence of solvent on the molecular structure. J Organometal Chem. 2009; 28:2096-2106.
  • Qiang J, Jiao Z, Xin S: Synthesis and crystal structure of nickel 3,5-pyridine dicarboxylates. Gaodeng Xuexiao Huaxue Xuebao 2010; 31:1496-5101.
  • Lin D, Kun-Miao W, Guang-Ke W, Rui-Bin F, Qu-Hua Z: Hydrothermal synthesis of two 3D lanthanide(III) organometallic polymers in the 3,5-dicarboxylicdicarboxylate system with different coordination architecture. Chinese J Structural Chem. 2010; 29:618-1624.
  • Dongbin D, Hui G, Yan B, Guoqiang Z: Synthesis, crystal structure and luminescent properties of one coordination polymer of copper (II) achieved from pyridine-3-5 dicarboxylate. Chem. Crystallography 2010; 40:332-336.
  • Song Y, Ji J, Han G, Zhang G, Han Z: Hydrothermal synthesis and structural characterization of a new coordination polymer [In(Pdc)(OH)(2,2’-Bipy)]In (H2Pdc=3,5-pyridinedicarboxylic acid, 2,2’bipyridine). Russian J. Coordination Chem. 2010; 36:113-116.
  • Zhang J, Chem S, Xiang S, Huang J, Chen L, Su C: Heterometallic coordination polymer gels based on a rigid, bifunctional ligand. Eur. J. 2011; 17:2369-2372.
  • Guo Z, Li Y, Yuan W, Zhu X, Li X, Cao R: Synthesis, structures, and characterizations of two new indium (III) compounds from 1D, In-OH-In-OH chains and pyridinedicarboxylic ligands. J. Inorg. Chem. 2008; 1326-1331.
  • Liu G, Xu Y, Ren X, Nishihara S, Huang R: Self-assembly of 3D 4d-4f coordination frameworks based on pyridine-3,5-dicarboxylic acid: synthesis, crystal structures and luminescence. Inorganica Chimica Acta 2010; 363:3727-3732.
  • Monmoton S, Lefebvre H, Costa-Torro F, Fradet A: Hyperbranched poly[bis(alkylene)pyridinium]s. Chem. Phys. 2008; 209:2382-2389.
  • Chekmeneva E, Hunter C, Packer M, Turega S: Evidence for partially bound states in cooperative molecular recognition interfaces. Am. Chem. Soc. 2008; 130:17718-17725.
  • Carraher CE, Morrison A, Roner MR, Moric A, Trang N: Synthesis and Characterization of Organotin Polyesters Derived from 3,5-Pyridinedicarboxylic Acid. Inorg. Organomet. Polym. 2014; 24:182-189.
  • Carraher C, Morrison A, Roner MR, Moric-Johnson A, Al-Huniti M, Miller L: Metallocene-containing polyesters from reaction of 3,5-pyridinedicarboxylic acid and metallocene dihalides and their preliminary ability to inhibit cancer call growth. JCAS 2015; 3:310-327.
  • Brickleband N, Godfrey S, Lane H, Mcauliffe C, Pritchard R, Moreno JM: Synthesis and structural characterization of R3AsX2 compounds (R = Me, Ph, p-FC6 H4 or p-MeOC6H4; X2 = Br2, I2 or IBr); dependency of structure of R, X and the solvent of preparation. Chem. Soc. Dalton Trans: Inorg. Chem. 1995; 23:3873-3876.
  • Carraher C, Roner MR, Dorestant J, Moric-Johnson A, Al-Huniti MH: Group VA Poly(amine Esters) Containing the Antibacterial Ampicillin. Inorganic Organometallic Polymeric Materials 2015; 25:400-410.
  • Carraher C, Roner MR, Ayoub M, Pham N, Moric A: Synthesis and Preliminary Cancer Activity of Chelidonic Acid Polyesters Containing the Triphenylarsenic, Triphenylantimony, and Triphenylbismuth International J Polym. Materials 2015; 64:311-319.
  • Carraher C, Roner MR, Thibodeau R, Johnson A: Synthesis, Structural Characterization, and Preliminary Cancer Cell Study Results for Poly(amine Esters) Derived from Triphenyl-Group VA Organometallics and Norfloxacin. Inorganica Chimica Acta 2014; 423:123-131.
  • Carraher C, Roner MR, Pham N, Moric A: Group VA Polyesters Containing Thiodiglycolic Acid-Synthesis and Preliminary Cancer Activity. Macromol. Sci, Part A 2014; 51:547-556.
  • Carraher C, Blum F, Nair M, Barot G, Battin A, Fiore T, Pellerito C, Scopelliti M, Zhao A, Roner MR, Pellerito L: Solid State Analysis of Metal-Containing Polymers Employing Mossbauer Spectroscopy, Solid State NMR and F EI TOF MALDI MS. Inorg. Organomet. Polym. 2010; 20:570-585.
  • Carraher C, Sabir TS, Carraher CL: Fundamentals of fragmentation matrix assisted laser desorption/ionization mass spectrometry. Inorganic and Organometallic Macromolecules. Springer, NY, 2008: 329-350.
  • Carraher C, Sabir T, Carraher CL: Fragmentation matrix assisted laser desorption/ionization mass spectrometry-basics. Polymer. Mater. 2006; 23:143-151.
  • Carraher C, Barot G, Battin A: Reactions between the matrix and ion fragments created from the MALDI MS or organotin-containing polymers. J. Polym. Mater. 2009; 26:17-31.
  • Carraher C, Roner MR, Carraher CL, Crichton R, Black K: Use of Mass Spectrometry in the Characterization of Polymers Emphasizing Metal-Containing Condensation Polymers. Macromol. Sci. 2015; 52:867-886.
  • Carraher C, Suresh V, Roner MR: Graphite as a Matrix for Organotin Polymers.  Polym. Mater. 2015; 32:151-168.
  • Carraher C, Siegmann-Louda D: Organotin macromolecules as anticancer drugs, in Macromolecules Containing Metal and Metal-Like Elements. Vol 3. Biomedical Applications. Wiley, Hobokin; 2004: 57-74.
  • Millich F, Carraher C: Interfacial Synthesis. Dekker, New York
  • Millich F, Carraher C: Interfacial Synthesis. Vol. II. Dekker, New York 1977.
  • Carraher C, Preston J: Interfacial synthesis. Dekker, New York 1977.
  • Carraher C, Barot G, Shahi K, Roner MR: Influence of DMSO on the Inhibition of Various cancer Cells by Water-Soluble Organotin Poly(ethers). JCAMS 2013; 1:294-304.
  • Carraher C: Polymer Chemistry, 9th Ed. Taylor and Francis, NY
  • Ohtaki H: Structural studies on solvationi and complexation of metal ions in nonaqueous solutions. Pure Appl. Chem. 1987; 59:1143-1150.
  • Gjevig Jenson K, Onfelt A, Wallin M, Lidumas V, Andersen O: Effects of organotin compounds on mitosis, spindel structure, toxicity, and in vitro microtubule assemble. Mutagenessis 1991; 6:409-416.
  • Corriu R, Dabosi G, Martineau M: The nature of the interactioni of nucleophiles such as HMPT, DMSO, DMF and Ph3PO with triorganohalo-silanes, -germanes, and -stannanes and organophosphorus compounds. Mechanism of nucleophile induced racmization and substitution at metal.   Organomet. Chem. 1980; 186:25-37.
  • Carraher C, Roner MR, Shahi K, Barot G: Structural Consideration in Designing Organotin Polyethers to Arrest the Growth of Breast Cancer Cells In Vitro. Materials 2011; 4, 801-815.
  • Carraher C, Roner MR: Organotin polyethers as potential biomaterials. Materials 2009; 2:1558-1598.
  • Roner MR, Shahi K, Barot G, Battin A, Carraher C: Preliminary results for the inhibition of pancreatic cancer cells by organotin polymers. Inorg. Ogranomet. Polym. 2009; 19:410-414.
  • Carraher C, Roner MR, Moric-Johnson A, Miller L, Barot G, Sookdeo N: Ability of Simple Organotin Polyethers to Inhibit Pancreatic Cancer. Macromol. Sci. 2015; 53:63-67.
  • Carraher C, Roner MR, Shahi K, Moric-Johnson A, Miller L, Barot G, Battin A, Trang N, Sookdeo N, Islam Z: Control of Breast Cancer Using Organotin Polymers. International Journal of Polymeric Materials and Polymeric Biomaterials. J. Poly. Mater. 2015; 64:800-814.
  • Barot G, Roner MR, Naoshima Y, Nago K, Shahi K, Carraher C: Synthesis, Structural Characterization, and Preliminary Biological Characterization of Organotin Polyethers Derived from Hydroquinone and Substituted Hydroquinones. Inorg. Organometal. Polym. 2009; 19:12-27.
  • Carraher C, Roner MR, Shahi K, Ashida Y, Barot G: Synthesis and Initial Cell Line results of Organotin Polyethers containing Diethylstilbestrol. Inorg. Organometal. Polym. 2008; 18:180-188.
  • Carraher C, Roner MR, Shahi K, Moric-Johnson A, Miller L, Barot G, Battin A, Trang N, Alhuniti M: Control of Prostate Cancer Using Organotin Polymers. Inorg. Organometal. Polym. 2015; 25:386-399.
  • Carraher C, Roner MR, Miller L, Shahi K, Trang N, Moric-Johnson A, Barot G, Battin A, Alhuniti M: Control of Colorectal Cancer Using Organotin Polymers. JCAMS 2014; 2:303-325.