Tumor dependence of observed thymidine index as a function of emulsion exposure.

The graphs of observed pulse thymidine index against duration of emulsion exposure for seven experimental tumor systems have been observed to exhibit significant differences; either they were parallel and displaced, or they were nonparallel. The immediate practical consequence of this phenomenon is that identical emulsion exposure times, other experimental conditions being equal, may not be sufficient to provide valid comparison of thymidine indices among tumors. A mathematical model is described which relates observed autoradiographic labeling index to the distribution of radioactive atoms among individual cells that initially incorporate labeled material. Two distributions for uptake are considered, and the results are compared. One model assumes uniform uptake, and the other assumes that the number of radioactive atoms among cells acquiring label follows a gamma distribution. The gamma-distributed uptake model fits the observed thymidine index data for the seven tumors and may be expressed as Lj(t) = a[1 - Qj(1 + bt) -u] where Lj(t) is the labeling index for emulsion exposure duration of t, Qj is a polynomial, and a, b, and u are adjustable parameters. The counting threshold for scoring a cell as labeled is j and for j = 1, Q1 = 1. The model indicates that the labeling index should increase with the duration of emulsion exposure and that this increase is related to the proportion of "lightly" labeled cells. The parameter a is the maximum expected labeling index, and an estimate of this parameter should theoretically be independent of emulsion exposure time.

Ambiguity of the thymidine index.

The observed thymidine indices of seven experimental tumor lines are compared as a function of duration of emulsion exposure. The effects of dose level of tritiated thymidine and background threshold are also evaluated. The results indicate that an arbitrary high background threshold discriminates against "lightly" labeled cells at short periods of exposure but that the chosen threshold becomes less critical with longer exposure. The observed thymidine index increases with increasing duration of emulsion exposure but appears to approach a plateau for all tumor systems. The "thymidine index curves" are significantly different for each tumor. There is an inverse relationship between the dose of tritiated thymidine and the duration of exposure required to recognize the same fraction of cells as labeled in a given tumor. Similar experimental conditions do not necessarily guarantee a valid basis for comparison of observed thymidine indices among tumors.

Diversity of penetration of anti-cancer agents into solid tumours.

Failure of anti-cancer agents to reach all clonogenic cells at cytotoxic concentrations is recognized as an important form of resistance in solid tumours. Subcutaneously implanted mammary adenocarcinoma 16/C was used to evaluate the intratumour distribution of five alkylating, bioreductive alkylating and intercalating agents and two radiation sensitizers. The agents were classified according to their in vivo distribution in well- and poorly-perfused tumour regions, as delineated by lissamine green. The classifications were: (1) distribution in direct proportion to the vascular supply; (2) uniform distribution to well- and poorly-perfused tumour regions; and (3) preferential retention in the poorly-perfused tumour regions. Our current state of knowledge did not allow reliable prediction of the classification based on chemical structure or mechanism of action.

Distribution of adriamycin in mice bearing mammary adenocarcinoma 16/C.

The response of solid mammary adenocarcinoma 16/C to treatment with Adriamycin is highly variable and ranges from growth under treatment to complete regression. Tumour and host factors were evaluated to determine the influence of each on the response. We determined that the concentration of Adriamycin in plasma and tumour was a function of tumour size and treatment history in mice bearing mammary adenocarcinoma 16/C. The plasma concentrations following a single dose of Adriamycin (10 mg/kg) increased in proportion to tumour mass without a concurrent increase in tumour concentration. When mice bearing large tumours (greater than 1.0 g) were treated with a multidose protocol, the plasma concentrations were higher and the tumour concentrations lower following the initial dose than following subsequent doses; in tumour-free mice, prior treatment with Adriamycin did not affect the plasma level achieved after a second dose. The magnitude of the decrease in plasma and increase in tumour concentrations was a function of the initial tumour size and the treatment schedule. The increase in tumour levels represented the sum of residual Adriamycin and drug bound as a result of the dose immediately prior to analysis. At the time of the initial treatment, the Adriamycin was distributed within each tumour in proportion to vascular perfusion. The percent of the tumour mass that was well-perfused appeared to increase with repeated treatments. The results indicate that the plasma concentration of Adriamycin did not necessarily reflect the tumour exposure in the mammary adenocarcinoma 16/C model. In hosts bearing mammary adenocarcinoma 16/C--or, possibly, other tumours that produce similar effects on the host--a low initial dose of Adriamycin might modify the distribution, possibly reduce the toxicity and allow escalation of subsequent doses with increased exposure of the tumour.

A conformationally defined 6-s-trans-retinoic acid isomer: synthesis, chemopreventive activity, and toxicity.

A conformationally defined retinoic acid analog (1) which contains a dimethylene bridge to maintain the 6-s-trans orientation for two terminal double bonds in the polyene chain was synthesized. A Reformatsky reaction was utilized to extend the polyene chain of the starting enone, which provided exclusively the 9Z-configuration for the intermediate aldehyde. A Horners-Emmons condensation with this aldehyde then produced retinoic acid analogs with both 9Z- and 9Z,13Z-configurations. An I2-catalyzed isomerization of the intermediate 9Z-aldehyde yielded the all-E-aldehyde, which was olefinated as above to yield the (all-E)- and (13Z)-retinoic acid analogs of 1. Each configurational isomer of 1 was evaluated for its ability to inhibit the binding of retinoic acid to CRABP (chick skin) and to inhibit the chemical induction of ornithine decarboxylase in mouse skin. In each assay (all-E)-1 was the most active isomer, and this activity was comparable to or better than that for (all-E)-retinoic acid. (all-E)-1 and (13Z)-1 were both shown to be equally effective as (13Z)-retinoic acid in suppressing the proliferation of human sebaceous cells in vitro. (all-E)-1 was further evaluated for its ability to prevent the induction of mouse skin papillomas and to induce signs of vitamin A toxicity in mice. The cancer chemopreventive activity of (all-E)-1 was comparable to that of (all-E)-retinoic acid, and the toxicity was comparable to or slightly better than that of the natural vitamin.

Conformationally defined retinoic acid analogues. 4. Potential new agents for acute promyelocytic and juvenile myelomonocytic leukemias.

We recently synthesized several conformationally constrained retinoic acid (RA) analogues [8-(2'-cyclohexen-1'-ylidene)-3, 7-dimethyl-2,4,6-octatrienoic acids with different alkyl substituents at 2' (R1) and 3' (R2) positions on the cyclohexene ring] (Muccio et al. J. Med. Chem. 1996, 39, 3625) as cancer chemopreventive agents. UAB8 (R1 = Et; R2 = iPr), which contains sufficient steric bulk at the terminal end of the polyene chain to mimic the trimethylcyclohexenyl ring of RA, displayed biological properties similar to those of RA. To explore the efficacy of this retinoid in acute promyelocytic leukemia (APL) and juvenile myelomonocytic leukemia (JMML), we evaluated UAB8 isomers in in vitro assays which measure the capacity of retinoids to inhibit aberrant myeloid colony growth from blood or bone marrow cells obtained from human JMML patients and in assays measuring the potential of retinoids to differentiate NB4 cells (an APL cell line). Both (all-E)- and (13Z)-UAB8 were 2-fold more active than RA in the NB4 cell differentiation assay; however, only (all-E)-UAB8 had comparable activity to the natural retinoids in the JMML cell assays. These results were compared to the biological effectiveness of a new retinoid, UAB30 [8-(3', 4'-dihydro-1'(2'H)-naphthalen-1'-ylidene)-3,7-dimethyl-2,4, 6-octatrienoic acid], which had different nuclear receptor binding and transactivational properties than UAB8. Relative to (all-E)-RA and (all-E)-UAB8, (all-E)-UAB30 bound well to RARalpha but did not activate transcription-mediated RARalpha homodimers, even though it was effective in RARbeta- and RARgamma-mediated transactivational assays. In APL assays, this retinoid had much reduced activity and was only moderately effective in JMML assays and in cancer chemoprevention assays.

Conformationally defined 6-s-trans-retinoic acid analogs. 3. Structure-activity relationships for nuclear receptor binding, transcriptional activity, and cancer chemopreventive activity.

We recently demonstrated that conformationally defined 6-s-trans-retinoic acid (RA) analogs were effective in the prevention of skin papillomas (Vaezi et al. J. Med. Chem. 1994, 37, 4499-4507) and selective agonists for nuclear receptor binding and activation (Alam et al. J. Med. Chem. 1995, 38, 2302-2310). In order to probe important structure-activity relationships, we evaluated a homologous series of four 6-s-trans-retinoids that are 8-(2'-cyclohexen-1'-ylidene)-3,7-dimethyl-2,4,6-octatrienoic acids with different substituents at 2' (R2) and 3' (R1) positions on the cyclohexene ring. UAB1 (R1 = R2 = H), UAB4 (R1 = R2 = Me), UAB7 (R1 = Me, R2 = iPr), and UAB8 (R1 = Et, R2 = iPr) contain alkyl R groups that mimic, to different extents, portions of the trimethylcyclohexenyl ring of RA. Both 9Z- and all-E-isomers of these retinoids were evaluated in binding assays for cellular retinoic acid-binding proteins (CRABP-I and CRABP-II), a nuclear retinoic acid receptor (RAR alpha), and a nuclear retinoid X receptor (RXR alpha). The all-E-isomers of UAB retinoids bound tightly to CRABPs and RAR alpha, the binding affinity of the all-E-isomer increased systematically from UAB1 to UAB8, and binding for the latter was comparable to that of all-E-RA. In contrast to RA, the (9Z)-UAB retinoids were at least 200-fold less active than the all-E-isomers in binding to RAR alpha. The (9Z)-UAB isomers exhibited increasingly stronger binding to RXR alpha, and (9Z)-UAB8 was nearly as effective as (9Z)-RA in binding affinity. The retinoids were also evaluated in gene expression assays mediated by RAR alpha and RXR alpha homodimers or RAR alpha/RXR alpha heterodimers. Consistent with the binding affinities, the (all-E)-UAB retinoids activated gene transciption mediated by RAR alpha homodimers or RAR alpha/RXR alpha heterodimers, while the (9Z)-UAB isomers activated only the RXR alpha homodimer-mediated transcription. The all-E- and 9Z-isomers of the UAB retinoids were further evaluated for their capacity to prevent the induction of mouse skin papillomas. When compared to RA, only the (all-E)-UAB retinoids containing bulky R1 and R2 groups were effective in this chemoprevention assay. (9Z)-RA displayed equal capacity as RA to prevent papillomas, while the 9Z-isomers of the UAB retinoids were much less effective. Taken together, these studies demonstrate that the cyclohexenyl ring substituents of 6-s-trans-UAB retinoids are important for their biological activities and that the chemopreventive effect of the all-E-isomers of these retinoids correlates well with their capacity to bind to RARs and activate RAR/RXR-mediated transcription.

Variability of tumor response to chemotherapy. II. Contribution of tumor heterogeneity.

The role of tumor-to-tumor variability in response to chemotherapy was investigated in mice bearing mammary adenocarcinoma 16/C treated with melphalan. Lissamine green, a triphenylmethane dye, was given systemically to delineate areas of perfusion in the tumors. The regions of low perfusion ranged from less than 10% to greater than 90% of the mass of individual tumors. The variation in perfusion was as large between bilateral tumors in a mouse as between tumors in different hosts. The presence of viable cells capable of continued growth in the regions of low perfusion was demonstrated by bioassay. Concentrations of melphalan following i.p. administration varied by as much as tenfold or more between regions of low and high perfusion. Concentrations of melphalan in the well-perfused regions were similar to plasma concentrations at 30 min after administration, but elimination from the plasma was more rapid. The levels of melphalan in the tumor were higher following the initial dose than following succeeding doses in a multiple dose schedule. The results indicate that tumor-to-tumor variations in perfusion and drug distribution are major factors in variable tumor response.

Distribution of nitroimidazoles and L-phenylalanine mustard in mammary adenocarcinoma 16/C tumors.

Using the triphenylmethane dye, lissamine green, as an indicator of blood perfusion, we have demonstrated that L-phenylalanine mustard (L-PAM) is differentially distributed in mice bearing mammary adenocarcinoma 16/C tumors. Following i.p. administration, concentrations of L-PAM in various regions of the tumors vary by as much as 10-fold or more between regions of low and high perfusion. Since the nitroimidazoles, metronidazole and misonidazole, increase the cytotoxicity of certain antitumor agents, these compounds were investigated for their ability to increase the distribution of L-PAM into tumor regions of low perfusion. Administration of metronidazole (400 mg/kg) or misonidazole (800 mg/kg) 1 h prior to L-PAM and lissamine green resulted in elevated plasma levels of L-PAM and increased concentrations of L-PAM in tumor regions of high perfusion. A slight increase in the normally low levels of L-PAM in tumor regions of low perfusion was observed but the increase was not statistically significant. In contrast to the uneven distribution of L-PAM, metronidazole and misonidazole were evenly distributed throughout plasma and tumor regions of both high and low perfusion. Bioassay of tumors following in vivo exposure to metronidazole and L-PAM indicated decreased viability in fragments from tumor regions of high perfusion, but not from tumor regions of low perfusion. These studies demonstrate that the nitroimidazoles increased L-PAM levels in plasma and in tumor regions of both high and low perfusion but did not induce a uniform distribution of L-PAM throughout the tumors. The nitroimidazoles may enhance the effectiveness of L-PAM as an antitumor agent by increasing the concentration of drug that reaches a tumor.

Variability of tumor response to chemotherapy. I. Contribution of host heterogeneity.

Host factors that might be associated with the variable response of tumors to effective chemotherapy were studied in B6C3F1 mice bearing transplanted mammary adenocarcinoma 16/C tumors and treated with melphalan. Tumor response ranged from regression to an unpalpable size to growth under treatment. That biochemical resistance of the cell population was not primarily responsible for the variability was demonstrated by passage of responsive and nonresponsive tumors into new hosts followed by treatment with melphalan. When the implanted subcutaneous tumor weighed 1.0 g or less (usually 12 to 13 days postimplant), both the plasma levels of melphalan and the variability in plasma levels were similar to those observed in tumor-free mice. With tumor progression beyond 1.0 g, an increase in mean plasma levels and in variability, but not in plasma half-life, was observed. A correlation between the dose of melphalan administered, the schedule, and the percentage of tumor responses was found. There was no correlation between the plasma levels in individual mice following a given dose of melphalan and subsequent tumor response. Also, there was no correlation between the plasma levels of melphalan in individual mice following the second, third or fourth treatment in a multiple-dose treatment schedule and the response of the tumor in that mouse to previous treatments. Prior therapy (1, 2 or 3 doses administered 4 days apart) either prevented the increase in plasma levels that occurred in mice bearing untreated advanced tumors or reduced the plasma level (and the variability) to approximately that found in tumor-free mice. Whether this was a direct result of the effects of melphalan on the host or an indirect result of tumor inhibition is not known. A similar study in tumor-free mice indicated that prior treatment had only minimal effects on subsequent plasma levels. These studies indicate that heterogeneity of the host was not a major factor in variable tumor response if therapy was initiated when the tumors weighed 1.0 g or less.

Biodistribution of radiolabeled lipid-DNA complexes and DNA in mice.

The tissue biodistribution and expression of [33P]DNA-1-[2-[9-(Z)-octadecenoyloxy]ethyl]]-2-[8](Z)-heptadece nyl]-3 -[hydroxyethyl]imidazolinium chloride (DOTIM):cholesterol complexes and 33P-radiolabeled DNA expressing chloramphenicol acetyl transferase (CAT; 4.7 kB) were studied after intravenous (iv) injection in ICR mice. Mice were injected with 200 microL of complex containing DNA at 3 mg/kg or DNA alone. One group received 8 microCi of radioactivity and were sacrificed at 5 and 20 min, and 1, 2, 4 and 24 h post-dose (n = 4/time point). A second group received the equivalent of 3.9 microCi of radioactivity and were sacrificed at 20 min, and 2 and 24 h for subsequent whole body autoradiographic analysis (WBA; n = 2/time point). The tissue distribution of intact DNA was assessed by Southern blot at 24 h post-dose, whereas the integrity of complexes and DNA incubated in heparinized whole blood was studied separately. In further studies, the time course of expression in lung tissue over a 48-h period was examined, and the relative lung-expression of purified open circular (OC) versus supercoiled (SC) DNA at 24 h was evaluated. Approximately 42% of the radioactivity was found in the lungs 5 min after injection and about half this percentage was found in the liver. By 2 h, only 5% remained in the lungs, but 48% was present in the liver. No other tissue accumulated >5% of the dose throughout the duration of the study. WBA radiograms confirmed the tissue distribution results and highlighted significant accumulation of radioactivity in bone over time. Southern Blot analysis demonstrated intact DNA in many tissues 24 h after dosing. In contrast, the majority of DNA incubated in blood was degraded within 2 h, although the complexes afforded some protection relative to DNA alone. The OC DNA expressed equivalently to SC DNA in lung tissue (OC = 1035 +/- 183 pg; SC = 856 +/- 257 pg/mg soluble protein, n = 6, mean +/- SEM) at 24 h, and detectable levels of CAT were present within 2 h of dosing (21.3 +/- 7.2 pg, n >/= 8, mean +/- SD). The results confirm that DNA-DOTIM:cholesterol complexes are initially deposited in the lungs after iv administration.

Effects of surgery on the cell kinetics of residual tumor.

Noncurative excision of a primary sc Lewis lung tumor performed on Day 7 or later results in an increase in the thymidine index and growth rate with minimal changes in the cell cycle parameters of the lung metastases. The stimulation of the lung nodules is accompanied by a small but consistent decrease in median lifespan. Sham surgery performed on Day 3 or later also results in a decrease in median lifespan and an increase in the thymidine index of the undisturbed primary tumor. Artifical metastases (10(6) cells iv) are inhibited by the presence of a second (sc) implant and the median lifespan of the doubly implanted mice exceeds that of mice bearing iv implants only. In mice bearing widely metastasized Lewis lung carcinoma, surgery alone may have a detrimental effect on life expectancy, but the residual tumor foci, stimulated to more rapid growth, should be appropriate targets for adjuvant chemotherapy.

The cell population kinetics and response to an S-phase specific agent of three transplantable colon tumor lines.

The relative cell population kinetics of three transplantable murine colon tumor lines (Colon 26, 36 and 38) with different histological and metastatic characteristics were studied in relation to the response of each line to an S-phase specific agent. The mean doubling times for the three lines between 0.1 and 1.0 g are similar (4.2 days) but marked differences are apparent in times to tumor appearance (0.1 g) and in median days to death. The length of the cell cycle is about one day and the length of the S-phase 10-11 hr for Colon 36 and 38. The length of the cell cycle in Colon 26 is difficult to estimate by conventional methods but probably exceeds 24 hr and the S-phase is 10-11 hr; [3H]TdR pulse labeling indices for Colon 36 and 38 decrease with time and tumor size from about 0.45 in 0.1 to 0.2 g tumors to about 0.33 at 3 g. The decrease in the [3H]TdR labeling index for Colon 26 is more pronounced (from about 0.38 at 0.1 g to 0.21 at 1.0 g). The shapes of the PLM curves and the [3H]TdR labeling index data are consistent with the observed sensitivity to an S-phase specific agent (Palmo-AraC, NSC 135962) in Colon 36 and the minimal response observed in Colon 26. Colon 38 is intermediate between Colon 36 and Colon 26 in kinetic properties and in response to the S-phase agent.

Studies of the growth, population kinetics, and host lethality of CD8F mammary adenocarcinoma.

First-generation transplants of spontaneous mammary adenocarcinomas in CDSF mice have been included in the tumor panel used for the screening of potential anticancer chemotherapeutic agents. Studies were undertaken to evaluate the growth and kinetic characteristics to provide a better understanding of the basic biology of the tumor system. The spontaneous mammary adenocarcinomas are highly variable in growth rate, and this characteristic is seen to a lesser extent in the transplanted tumors. The doubling time of the transplants is usually, but not always shorter than the doubling time of the donor tumor at a similar mass. The median length of the cell cycle obtained from pooled data from first-generation transplants of four donor tumors is 15.6 hours with a median S phase of 9.5 hours. The thymidine indices (TI) ranged from a mean of 0.28 at 100 mg to a mean of 0.20 when tumors ranged from 3 to 6 g. The mean TI of tumors of all sizes is 0.27. The cell cycle parameters and TIs appear to be characteristic of the tumor system and apparently do not vary with characteristics of the donor tumor. These studies also indicate that these kinetic parameters are not responsible for the variation in volume doubling time of the transplanted tumors.

Basic and therapeutic trial results obtained in the spontaneous AK leukemia (lymphoma) model-end of 1971.

Basic and therapeutic trial results obtained in the spontaneous AK leukemia (lymphoma) model have been brought together for comparison with available information on the much used transplanted murine leukemia models and human leukemias and lymphomas. The etiologic agent for "spontaneous" AK lymphoma is an RNA virus present at birth in AKR mice. Lymphoma cells first appear in the thymuses of animals at 5-->12 months of age. The time lapse between the first appearance of viable lymphoma cells in the thymus and clinical diagnosis (eg, with about 10(9) widely disseminated viable plus nonviable lymphoma cells in the host) is about 1 month. Thus, the overall doubling time of lymphoma cells before diagnosis is about 1 day. This estimate is compatible with the doubling time of relatively small numbers of first-passage lymphoma cells, assay data on the rate of repopulation of viable lymphoma cells after therapeutic reduction, and the median intermitotic time of dividing lymphoma cells (ie, 0.6 day). In general, the cytokinetic parameters of advanced spontaneous AK lymphoma cell populations are more like those observed in advanced human leukemias than are those of early L1210 leukemia. This paper presents assay data on the reduction in viable spontaneous AK lymphoma cells after treatment with a variety of agents, and the rate of cell repopulation after cessation of treatment. Extensive therapeutic trial data indicate that cyclophosphamide is presently the most effective single agent against spontaneous AK lymphoma, with arabinosylcytosine or palmO-ara-C a close second. Daunomycin, 5-fluorouracil, the nitrosoureas, vincristine, methotrexate, and dexamethasone provided moderate increases in host survival time. The combination of vincristine plus prednisone was a good remission inducer but the median survival time after cessation of treatment was shorter than that observed for cyclophosphamide or palmO-araC. The best responses observed to date with two-drug combinations appear better on several scores than the best that have been observed with single drugs. The best overall responses observed to date with two-drug combinations were with palmO-ara-C plus methyl-CCNU, cyclophosphamide plus methyl-CCNU, and palmO-ara-C plus cyclophosphamide. Some three- and four-drug combinations have provided better therapeutic responses than have been observed with single agents but not significantly better than those obtained with two-drug combinations. Splenomegaly assays carried out immediately after cessation of treatment and 60 days and longer after cessation of treatment, suggest that eradication of all viable lymphoma cells is being achieved in some animals by combination chemotherapy; however, such animals eventually die of lymphoma, presumably as a result of the reinduction of a second lymphoma cell population. The requirements for permanent "cure" of spontaneous lymphoma in AKR mice include eradication of all viable lymphoma cells and prevention of reinduction. Two major differences between early L1210 leukemia and clinically diagnosed spontaneous AK lymphoma are the degree of disease advancement at the time therapy is usually started (and associated cytokinetic differences) and the reinduction problem in AKR mice. Spontaneous AK lymphoma is relatively more advanced at diagnosis than is acute leukemia in man (ie, with respect to nearness of the host to death), and it is presumed that the reinduction problem in AKR mice is more acute and more prevalent than in human neoplastic disease.

Murine toxicology and pharmacology of UAB-8, a conformationally constrained analog of retinoic acid.

(2E, 4E, 6E)-8-[3'-Ethyl-2'-(1-methylethyl)-2'-cyclohexen-1'-ylidene] -3, 7-dimethyl-2,4,6-octatrienoic acid (UAB-8) has potent activity in preventing papillomas on the skin of mice similar to that determined in a previous study for the homolog containing one less carbon atom. To evaluate the toxicological profile for UAB-8, relative to all-trans-retinoic acid (RA), female mice were dosed by oral gavage for 29 days with amounts of 0.05, 0.1, or 0.2 mmol/kg/day. For the two compounds, the effects on body weights were similar. Mice dosed with UAB-8, however, had a lower incidence of clinical signs of toxicity (alopecia, scaly skin, and limping). At necropsy, bone fractures, skin abnormalities, and splenomegaly were observed in some mice dosed with RA but not in any dosed with UAB-8. Lymph node hyperplasia was noted in some mice dosed with either dose of RA but only in those dosed with the highest dose of UAB-8. All dose levels of RA produced microscopic lesions in the bones of mice; only the highest dose of UAB-8 had this effect. RA and UAB-8 had similar effects on chondrogenesis in cultures of cells from mouse limb buds, an indication of comparable teratogenic effects. For mice dosed i.v. (10 mg/kg), there was a saturated phase of elimination of RA from plasma (Km = 0.61 microgram/ml and Vmax = 2572 micrograms/hr); no such phase was noted when UAB-8 was administered. UAB-8 had values for t1/2 alpha and t1/2 beta of 0.47 and 17.1 hr, respectively. Relative to RA, UAB-8 has a favorable toxicological profile and different pharmacokinetics.