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Biomedical subjects

T M Allen

Publications and source records attributed to T M Allen.

At least 73 records · Page 4Linked to original sources
PubMed1997

Immunodominance of a single CTL epitope in a primate species with limited MHC class I polymorphism.

MHC class I molecules play a crucial role in immunity to viral infections by presenting viral peptides to cytotoxic T lymphocytes. One of the hallmarks of MHC class I genes in outbred populations is their extraordinary polymorphism, yet the significance of this diversity is poorly understood. Certain species with reduced MHC class I diversity, such as the cotton-top tamarin (Saguinus oedipus), are more susceptible to fatal viral infections. To explore the relationship between this primate's limited MHC class I diversity and its susceptibility to viruses, we infected five cotton-top tamarins with influenza virus. Every tamarin recognized the same immunodominant CTL epitope of the influenza nucleoprotein. Surprisingly, this nucleoprotein peptide was bound by Saoe-G*08, an MHC class I molecule expressed by every cotton-top tamarin. Two tamarins also made a subdominant response to an epitope of the matrix (M1) protein. This peptide appeared to be bound by another common MHC class I molecule. With the exception of an additional subdominant response to the polymerase (PB2) protein in one individual, no other influenza-specific CTL responses were detected. In populations or species with limited MHC class I polymorphism like the cotton-top tamarin, a dependence on shared MHC class I molecules may enhance susceptibility to viral infection, since viruses that evade MHC class I-restricted recognition in one individual will likely evade recognition in the majority of individuals.

Animals↗
PubMed1997

Liposomes. Opportunities in drug delivery.

Liposomal drug delivery systems markedly alter the biodistribution of their associated drugs by delaying drug clearance, retarding drug metabolism, decreasing the volume of distribution, and shifting the distribution in favour of diseased tissues with increased capillary permeability. This increases the therapeutic indices of the associated drugs, by increasing the drug concentration in solid tumours and regions of infection, and reducing the drug concentration in normal tissues. Three liposomal formulations have been approved for clinical use.

Antineoplastic Agents↗
PubMed1997

Synthesis and cytotoxic evaluation of some carbohydrazones and thiocarbohydrazones of various unsaturated ketones and related Mannich bases.

A number of 1-aryl-1-penten-3-ones 1 and related Mannich bases 2 were synthesized and converted either directly or indirectly to the corresponding biscarbohydrazones 3 and 4. Reaction of representative unsaturated ketones with thiocarbohydrazide led to the formation of only the monothiocarbohydrazones 5. The cytotoxicity of these compounds against murine P388 and L1210 cells, human T-lymphocytes and human tumour cell lines was undertaken in order to ascertain their bioactivity and review these data in the light of the theory of sequently cytotoxicity whereby the carbohydrazones would be predicted to be more than twice as cytotoxic as the ketones and Mannich bases from which they were derived. In general, conversion of 1 into 3 was accompanied by a lowering of cytotoxicity. On the other hand, in general the compounds in series 4 were significantly more potent cytotoxic agents than the analogues 2; in fact, some compounds displayed activity comparable with or exceeding that of melphalan. Thus the biscarbohydrazones 4 represent a novel group of cytotoxic agents serving as lead molecules for subsequent development.

Animals↗
PubMed1996

Speciation of arsenic oxides using laser desorption/ionization time-of-flight mass spectrometry.

Positive and negative ion mass spectra of arsenic trioxide (As2O3) and arsenic pentaoxide (As2O5) have been obtained by single-step laser desorption/ionization time-of-flight mass spectrometry. Pulsed UV radiation at 266 nm was used for the simultaneous desorption and ionization of the solid sample. High-mass cluster ions that are unique to the oxidation state of each oxide sample appear in the negative ion mass spectra. The As2O3 produces As3O5-, while the As2O5 yields As3O8-. The formation of unique negative cluster ions presents the capability for arsenic oxidation state speciation by laser desorption/ionization mass spectrometry. The ability of time-of-flight mass spectrometry to examine the relative amounts of each arsenic oxide present in a series of mixtures is discussed. Application of our speciation technique to a model incinerator sample is demonstrated.

Arsenic Trioxide↗
PubMed1996

Insertion of poly(ethylene glycol) derivatized phospholipid into pre-formed liposomes results in prolonged in vivo circulation time.

Transfer of MPEG(1900)-DSPE from micellar phase to pre-formed liposomes imparts long in vivo circulation half-life to an otherwise rapidly cleared lipid composition. MPEG(1900)-DSPE transfers efficiently and quickly in a time and temperature dependent manner. There is negligible content leakage and a strong correlation between assayed mol% MPEG(1900)-DSPE, liposome diameter increase, and pharmacokinetic parameters such as distribution phase half-life. Since a biological attribute (liposome clearance rate) can be modified by the insertion process, it suggests a simple and economical way to impart site-specific targeting to a variety of liposome delivery systems. This method is also a convenient way to measure the 'brush' thickness of such conjugates directly.

Animals↗
PubMed1996

The T-cell receptor beta chain-encoding gene repertoire of a New World primate species, the cotton-top tamarin.

The New World primate, the cotton-top tamarin (Saguinus oedipus), expresses major histocompatibility complex (MHC) class I molecules with limited diversity. The uniqueness of the cotton-top tamarin MHC class I loci may contribute to this species' unusual susceptibility to viral infections and high incidence of ulcerative colitis. As a prelude to examining the effect of this limited MHC class I diversity on the tamarin CD8(+) T-cell receptor (TCR) repertoire, we identified expressed tamarin TCR beta chain (TCRB) cDNAs by anchored and inverse polymerase chain reaction. Sequence alignments and phylogenetic comparisons with human and rhesus macaque sequences identified homologues of 21 human variable (V) gene families. Only single variable region genes were identified in each of these tamarin VB families, with the exception of the VB 5, 9, and 13 families which were comprised of two or three distinct members. The multiple genes within these three VB families do not appear to have separate human homologues, but rather aligned equally well to a single human gene from their respective VB families. These genes appear to have arisen, therefore, by duplication of certain VB genes in the tamarin ancestors following their divergence from the lineage leading to Old World primates and hominoids. Homologues of 12 of the 13 human joining (J) region genes were also identified in the tamarin. Comparison of the proportion of nonsynonymous (pN) and synonymous (pS) substitutions occurring per site within tamarin variable region genes demonstrated a reduction in pN in the framework regions compared with pN in the presumed MHC contact regions (CDR1 and CDR2). Taken together, these findings illustrate that the TCR beta chain-encoding genes of the cotton-top tamarin are similar in structure and degree of complexity compared with their Old World primate and human counterparts.

Amino Acid Sequence↗
PubMed1996

Evaluation of blood clearance rates and biodistribution of poly(2-oxazoline)-grafted liposomes.

Two amphipatic polymers of the poly(2-oxazoline) family, poly(2-methyl-2-oxazoline) (PMOZ) and poly(2-ethyl-2-oxazoline) (PEOZ), were synthesized with the carboxylic group positioned at either the initiation or termination ends of the polymer chains. Distearoylphosphatidylethanolamine was covalently linked to the carboxyl groups of the polymers, resulting in conjugates which incorporate readily into liposomes. Systematic evaluation of plasma clearance kinetics and biodistribution of liposomes containing hydrogenated soy phosphatidylcholine, cholesterol, and 5 mol % the polymer-lipid conjugates in mice revealed the following. Both polymers, PMOZ and PEOZ, exhibited long plasma lifetimes and low hepatosplenic uptake. PMOZ was more effective at decreasing blood clearance rates than PEOZ. The best results, which were quantitatively comparable to the results obtained with the optimized preparations of methoxypolyethylene glycol(PEG)-2000-grafted liposomes, were obtained with formulations containing PMOZ of molecular weight 3260.

Animals↗
PubMed1995

Attachment of antibodies to sterically stabilized liposomes: evaluation, comparison and optimization of coupling procedures.

Several coupling methods for binding antibodies (Ab) to liposomes have previously been developed. We were interested in examining if some of these methods would be suitable for attaching Ab to long-circulating formulations of liposomes (SL), sterically stabilized with poly(ethylene glycol) (PEG). We studied three 'classical' coupling methods in which Ab was attached at the bilayer surface of SL, and two new coupling methods in which Ab was attached at the PEG terminus. Parameters examined including binding efficiency, antibody surface density, the ability of the immunoliposomes to remote-load the anticancer drug doxorubicin, and the specific binding of the resulting immunoliposomes to target cells. The non-covalent biotin-avidin coupling method resulted in low Ab densities at the cell surface, as did a coupling in method in which maleimide-derivatized Ab was attached to the liposome surface through a thiolated phospholipid incorporated into the liposomes. The low levels of Ab achieved in these method was likely due to interference by PEG with the access of the Ab to the liposome surface. However, when a maleimide-derivatized Ab was coupled to thiolated PEG, moving the coupling reaction away from the liposome surface, very high coupling efficiencies were achieved, and these immunoliposomes achieved good specific binding to their target cells. Oxidizing the Fc region of the Ab and coupling it to the PEG terminus through a hydrazone bond was a less efficient coupling method, but had the advantage of retaining Ab orientation. Efficient remote-loading of doxorubicin was found for immunoliposomes in which Ab was attached at the PEG terminus.

Adenocarcinoma↗
PubMed1995

A new strategy for attachment of antibodies to sterically stabilized liposomes resulting in efficient targeting to cancer cells.

The development of long-circulating formulations of liposomes (S-liposomes), sterically stabilized with lipid derivatives of poly(ethylene glycol) (PEG), has increased the likelihood that these liposomes, coupled to targeting ligands such as antibodies, could be used as drug carriers to deliver therapeutic drugs to specific target cell populations in vivo. We have developed a new methodology for attaching monoclonal antibodies to the terminus of PEG on S-liposomes. A new end-group functionalized PEG-lipid derivative pyridylthiopropionoylamino-PEG- distearoylphosphatidylethanolamine (PDP-PEG-DSPE) was synthesized for this purpose. Incorporation of PDP-PEG-DSPE into S-liposomes followed by mild thiolysis of the PDP groups resulted in formation of reactive thiol groups at the periphery of the lipid vesicles. Efficient attachment of maleimide-derivatized antibodies took place under mild conditions even when the content of the functionalized PEG-lipid in S-liposomes was below 1% of total lipid. The resulting S-immunoliposomes showed efficient drug remote loading, slow drug release rates and increased survival times in circulation compared to liposomes lacking PEG. When antibodies recognizing several different tumor-associated antigens were coupled to the PEG terminus of S-liposomes a significant increase in the in vitro binding of liposomes to the target cells was observed. The binding of S-immunoliposomes containing entrapped doxorubicin to their target cell population resulted in increased cytotoxicity compared to liposomes lacking the targeting antibody.

Antibodies, Neoplasm↗
PubMed1995

Pharmacokinetics and anti-tumor activity of vincristine encapsulated in sterically stabilized liposomes.

Vincristine is used clinically for the treatment of various types of cancer. Recent significant therapeutic improvements obtained by entrapping anthracyclines in sterically stabilized liposomes raised the question whether the therapeutic index of vincristine can be similarly increased by formulation into such long-circulating liposomes. Encapsulation of vincristine in sterically stabilized liposomes (SL-VCR) prolonged the drug's distribution phase plasma half-life in rats from 0.22 to 10.5 hr. There was no significant difference in LD50 (> < or = 2.5 mg/kg, i.v.), but mice given sublethal doses of SL-VCR experienced significantly less weight loss than those given the same dose of free drug. Compared to free drug, SL-VCR was most effective against i.p. or s.c. implanted tumors. However, i.v. tumor inoculation nullified the therapeutic advantage of encapsulation. A single i.v. 2 mg/kg dose of SL-VCR increased the life span of mice bearing i.v. implanted P388 cells by only 44%, while the life span of i.p. P388 implanted mice was increased by 199%. In an s.c. implanted murine colon carcinoma, multiple doses of free drug did little to slow the growth of the tumors, but SL-VCR was able to produce long-term survivors in several dose regimens. These results indicate that prolonged circulation time increases the therapeutic index of VCR entrapped in liposomes against s.c. or i.p. implanted tumors, but does not improve the drug's activity against rapidly growing i.v. disseminated leukemias.

Animals↗
PubMed1995

Dual role of melanins and melanin precursors as photoprotective and phototoxic agents: inhibition of ultraviolet radiation-induced lipid peroxidation.

Ultraviolet radiation (UVR) is one of the risk factors for skin cancer and the main inducer of melanin pigmentation, the major protective mechanism of mammalian skin against radiation damage. The melanin pigments, eumelanin and pheomelanin, are likely to be important in protection against UVR, but their precursors are generally considered as phototoxic. The available data suggest DNA damage as the mechanism of phototoxicity. However, the effect of melanin precursors on membrane damage through lipid peroxidation, another important and probably more relevant (from the point-of-view of the melanosomal confinement of these molecules) mechanism of phototoxicity, not known. As a model system for UVR-melanin-membrane interactions, we irradiated liposomes in the presence of eumelanin, pheomelanin and two of their major precursors, 5,6-dihydroxyindole (DHI) and 5-S-cysteinyldopa (SCD). The presence of the two melanin precursors substantially reduced the formation of lipid peroxidation products resulting from UVR exposure. The antioxidant activity of the melanin precursors was diminished under strong prooxidant conditions (presence of Fe3+). These results suggest that melanin precursors may have an important role in the protection of skin against the harmful effects of UVR including photocarcinogenesis.

Animals↗
PubMed1995

Synthesis and cytotoxic evaluation of some Mannich bases of alicyclic ketones.

A number of Mannich bases of alicyclic ketones containing one and two basic centres were prepared in order to evaluate the theory of sequential cytotoxicity and develop structure-activity relationships in these series of compounds. The compounds were evaluated in vitro against murine P388 D1 lymphocytic leukemia cells. The data generated supported the theory of sequential cytotoxicity and in general, compounds containing alicyclic rings of five and six carbon atoms possessed greater activity than the corresponding dodecyl analogues. Those Mannich bases containing dialkylamino groups were associated with greater cytotoxicity than related compounds possessing a basic heterocycle. Calculations of the atomic charges of the enone groups from selected compounds afforded some rationalization for the cytotoxic screening results.

Animals↗
PubMed1995

Synthesis and cytotoxic evaluation of mesna adducts of some 1-aryl-4,4-dimethyl-5-(1-piperidino)-1-penten-3-one hydrochlorides.

Reaction of 1-(4-bromophenyl)-4,4-dimethyl-5-(1-piperidino)-1-penten-3-one hydrochloride (1f) with sodium 2-mercaptoethanesulphonate (mesna) gave rise to the thiol adduct. 3. Recrystallization of this compound led to the formation of the corresponding zwitterion 4f. A series of analogues of 4f were prepared and the structure of a representative compound was confirmed by X-ray crystallography. In general, the thiol adducts had similar activity towards P388 cells and human tumour cell lines as the precursor enones 1 although greater selectivity to malignant diseases was found with the thiol adducts. A stability study of representative compounds conducted by 1H NMR spectroscopy revealed that the thiol adducts decomposed in solution. In one case regeneration of the ketone was noted while for the other compounds, the decomposition products were not identified.

Animals↗
PubMed1994

Synthesis and cytotoxic evaluation of some cyclic arylidene ketones and related oximes, oxime esters, and analogs.

A number of arylidene derivatives of alicyclic ketones and some corresponding oximes, oxime esters, and related compounds were prepared as candidate cytotoxic agents. All of the compounds were evaluated against murine L1210 lymphoid leukemia cells. In general, cytotoxicity was greatest with the alpha,beta-unsaturated ketones and diminished with the oximes, and the oxime esters had little or no activity in this screen. When the same compounds were examined in both the in vitro L1210 and P388 leukemia screens, in the majority of cases the L1210 cells were more sensitive to these derivatives. Over half of the compounds prepared were evaluated against approximately 55 human tumors in vitro and showed selective toxicity toward one or more groups of neoplastic diseases, particularly leukemia. Some correlations between structure and bioactivity were discerned. The cytotoxicity screening and stability studies of representative compounds suggested that the ketones, oximes, and oxime esters were stable under the conditions of bioevaluation. X-ray crystallography of four representative compounds revealed structural features associated with cytotoxicity which may be considered in the design of future candidate cytotoxins.

Animals↗
PubMed1994

Cytotoxic evaluation of some 3,5-diarylidene-4-piperidones and various related quaternary ammonium compounds and analogs.

A number of 3,5-diarylidene-4-piperidones (1) and some related quaternary ammonium salts (5) as well as closely related analogs were prepared principally as candidate cytotoxic agents in two screens. The first test system used an average of 54 human tumor cell lines from eight neoplastic diseases, namely leukemia, melanoma, colon, non-small-cell lung, small-cell lung, central nervous system, ovarian, and renal cancers. Selective toxicity was demonstrated by some of the compounds, especially toward leukemia. The second screen used L1210 lymphoid leukemia cells. In general, the compounds were less cytotoxic than the reference drug melphalan in both screens. Linear plots were made between the Hammett (sigma), fragment (f), and molar refractivity (MR) constants of the nuclear substituents in series 1 and 5 with the IC50 figures of both the human tumor cell lines and L1210 cells. Evaluation against the human tumor cell lines revealed that increases in the f values were correlated with elevation of cytotoxicity in both series 1 and 5; MR constants were also important in series 5. In the L1210 screen, sigma and MR constants were positively correlated with cytotoxicity. X-ray crystallography was undertaken on 3,5-bis-[[4'-(methylthio)phenyl]methylene]-1-methyl-4-piperidone methiodide (5d), which had significant cytotoxicity, and 3,5-bis(4-pyridylmethylene)-1-methyl-4-piperidone methiodide (6), which was virtually inactive in both screens.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗