The maytansinoid drug DM1 is 100-to 1000- Colorectal cancer is one of the most common malignancies and is among the leading causes of death from cancer. Surgical resection is the primary treatment modality for these tumors, but about half of all patients will die of disseminated disease (1). Because of the high incidence and poor prognosis of patients with metastatic disease, successful treatment of colorectal cancer requires effective systemic therapy in addition to surgery, either as adjuvant treatment to surgery or for primary treatment of those 25% of all patients for whom surgery alone cannot achieve a complete response (2). Unfortunately, the conventional systemic treatment options for colon cancer, including radiation therapy, chemotherapy, and immunotherapy, have limited efficacy (3, 4). To date, 5-fluorouracil (5-FU) has served as the standard cytostatic drug for adjuvant therapy after surgery. However, the overall response rate to 5-FU is less than 25%, and the treatment has not significantly improved patient survival (1-3). Although the improved regimen of 5-FU plus levamisole in the adjuvant setting has proven to be more effective in patients with stage II and III colorectal cancers, the estimated reduction in the mortality rate is still less than 30% (2, 5). Thus, there is an urgent clinical need for new agents with greater efficacy.Conventional chemotherapeutic agents are limited in their therapeutic effectiveness by severe side effects due to their poor selectivity for tumors. The development of monoclonal antibodies against specific tumor antigens made it possible to think of enhancing the selectivity of anticancer drugs by a targeted delivery approach. However, several such reported attempts using monoclonal antibodies and the anticancer drugs doxorubicin (6), methotrexate (7), and Vinca alkaloids (8), have been largely unsuccessful. These antibody-drug conjugates were only moderately potent and usually less cytotoxic than the corresponding unconjugated drugs. In fact, antigenspecific cytotoxicity toward cultured tumor cells was rarely demonstrated (6-8). In vivo therapeutic effects with these conjugates in tumor xenograft animal models were, in general, observed only when the treatments were commenced before the tumors were well established (8) or when exceedingly large doses (up to 90 mg/kg, drug equivalent dose) were used (6). It is, therefore, not surprising that in human clinical trials, no significant antitumor effects were observed with these agents (9, 10). Indeed, the peak circulating serum concentrations of conjugates were only in the same range as their in vitro IC50 values and, thus, capable of eliminating at best only about 50% of tumor cells.These observations have led us (11, 12) and others (13, 14) to conclude that the previous attempts at delivering therapeutic doses of cytotoxic drugs via monoclonal antibodies have met with little success in clinical trials because of inappropriate choices of drug. We concluded that immunoconjugates must be composed of drugs possessing ...

Compressive forces of physiological magnitude (60 grams per square centimeter) reduce the adenosine 3',5'-monophosphate and guanosine 3',5'-monophosphate content of the epiphyses of tibiae from 16-day-old chick embryos. An equivalent hydrostatic pressure applied directly to cells isolated from this tissue also affects cyclic nucleotide accumulation. The tissue response is uniform throughout the epiphysis, whereas the cell response varies according to the area of origin.

Erythrocyte ghosts loaded with ' 25 1-labeled proteins were fused with confluent monolayers of IMR-90 fibroblasts using polyethylene glycol . Erythrocyte-mediated microinjection of ' 25 1-proteins did not seriously perturb the metabolism of the recipient fibroblasts as assessed by measurements of rates of protein synthesis, rates of protein degradation, or rates of cellular growth after addition of fresh serum.A mixture of cytosolic proteins was degraded after microinjection according to expected characteristics established for catabolism of endogenous cytosolic proteins . Furthermore, withdrawal of serum, insulin, fibroblast growth factor, and dexamethasone from the culture medium increased the degradative rates of microinjected cytosolic proteins, and catabolism of long-lived proteins was preferentially enhanced with little or no effect on degradation of shortlived proteins . Six specific polypeptides were degraded after microinjection with markedly different half-lives ranging from 20 to 320 h . Degradative rates of certain purified proteins (but not others) were also increased in the absence of serum, insulin, fibroblast growth factor, and dexamethasone .The results suggest that erythrocyte-mediated microinjection is a valid approach for analysis of intracellular protein degradation. However, one potential limitation is that some microinjected proteins are structurally altered by the procedures required for labeling proteins to high specific radioactivities. Of the four purified proteins examined in this regard, only ribonuclease A consistently showed unaltered enzymatic activity and unaltered susceptibility to proteolytic attack in vitro after iodination .Intracellular protein degradation is a fundamentally important process occurring in all organisms from bacteria to humans (5,20,23,24,53) . The continued breakdown and replacement of proteins allows the cell to regulate concentrations of specific enzymes as well as to alter overall protein content in response to changing physiological demands. The major areas of current study in protein degradation can be divided into three broad topics : (a) the influence of polypeptide structure on protein half-lives (20, 23), (b) the physiological regulation of protein degradative rates (5,20,24), and (c) the mechanisms by which proteins are degraded within cells (5,24,53) . Despite considerable recent progress in each ofthese areas, many ofthe major questions in this field of research remain unanswered.Microinjection offers several advantages for analysis of protein degradation over more conventional approaches in which endogenous cellular proteins are radiolabeled and their deg- radative rates determined (I1, 34, 51). Microinjection of mammalian cells in culture has been achieved using microneedles (6,55) or by inducing fusion of the recipient cell with erythrocyte ghosts containing the protein to be microinjected (6,34,51) . Most studies of protein degradation have used erythrocytemediated microinjection because sufficient numbers of cells can be microinject...

External oscillating electric fields (1166 volts per centimeter, 5 hertz) enhanced the incorporation of [3H] thymidine into the DNA of chondrocytes isolated from the proliferative layer of embryonic (16 days) chick epiphysis. Verapamil or tetrodotoxin at 10(-6)M concentrations completely blocked the electric field effect. Tetracaine reduced the incorporation of [3H] thymidine in both control and electrically stimulated cells. The findings support the hypothesis that Na and Ca2 fluxes generated by the electrical perturbation trigger DNA synthesis in these cells.

Folate receptor is over-expressed in a variety of carcinomas. To design a cytotoxic drug that would selectively target these carcinomas, we synthesized folate-maytansinoids. These drugs showed high affinity toward folate receptor, appeared to enter cells exclusively via the folate receptor-mediated caveolar pathway and displayed high cytotoxic potency (in the range of 10 ؊11 to 10 ؊10 M) and remarkable selectivity for folate receptor-expressing carcinoma cell lines. Folatemaytansinoids represent a new class of tumor-specific agents in which the targeting and the cytotoxic function can be altered independently. Int. J. Cancer 73:859-864, 1997. Wiley-Liss, Inc.The therapeutic effectiveness of anti-cancer drugs in current clinical use is limited by their low selectivity for tumors and associated high systemic toxicity. Enormous research efforts, therefore, were undertaken to develop tumor-selective drugs by conjugating a toxin or a cytotoxic drug to an antibody or a hormone that binds selectively to cancer cells (Pastan et al., 1992;Pietersz and Krauer, 1994). One cell surface-expressed molecule that is very attractive for targeting cytotoxic agents is folate receptor because it is greatly over-expressed in a variety of carcinomas and has a highly restricted distribution of expression in normal tissues (Stein et al., 1991;Franklin et al., 1994;Garin-Chesa et al., 1993; Weitman et al., 1992a,b;Ross et al., 1994). The first vehicles used to deliver cytotoxic moieties to folate receptor-expressing cells were anti-folate receptor monoclonal antibodies (MAbs) (Cogliati et al., 1991;Coney et al., 1994). Antibodies are large proteins whose targeting capacity and therapeutic usefulness may be hampered by their poor tumor penetration (Jain, 1989;Fujimori et al., 1989;Cassidy et al., 1993;Debinski and Pastan, 1995) and immunogenicity (Ghetie and Vitetta, 1994). These problems could be avoided by using small molecules for targeting drugs. Folate receptor binds its natural low m.w. ligand, folic acid, with high affinity (K d in the range of 1 nM or less [Kamen, 1987;Sirotnak et al., 1987]), and only low concentrations of folic acid derivatives (collectively called folates) that can bind to the receptor are found in blood (Kamen, 1987;Sirotnak et al., 1987). Therefore, it was proposed that extraneously administered folic acid could compete easily with the endogenous folates and serve as a targeting vehicle. Folic acid would preferentially target folate receptor and not folate transporter since the latter binds folic acid with an affinity that is at least 10 4 -fold lower (Kamen, 1987;Sirotnak et al., 1987) than the affinity toward folate receptor. Folic acid was conjugated to protein toxins (Leamon and Low, 1992;Leamon et al., 1993). These agents, however, also may be prone to the limitations typical for large protein-based therapeutic agents. We wished to design a low m.w. cytotoxic drug targeted to folate receptor. To achieve this goal, we synthesized the maytansinoid DM1, a highly cytotoxic drug that inhibits the polymerizat...

RNase A introduced into the cytoplasm of IMR-90 human -diploid fibroblasts by erythrocyte-mediated microinjection is degraded with a half-life of =75 hr in the presence of fetal bovine serum. In response to serum deprivation the degradative rate of microin'ected RNase A is enhanced 2-fold. RNase S protein (amino. acids 21-124) is degraded with a half-life similar to that of RNase A in the presence of serum, but its catabolism is not increased during serum withdrawal. Reconstitution of RNase S protein with RNase S peptide (amino acids 1-20) restored full enzymatic activity to the S protein as well as the ability of fibroblasts to increase its catabolism during serum deprivation. Finally, RNase S peptide microinjectedalone shows the full 2-fold increase in degradative rate during serum withdrawal. These results show that recognition of RNase A for enhanced breakdown during serum deprivation is based on some feature of its amino-terminal 20 amino acids. Furthermore, our results indicate that the enhanced protein catabolism during serum. deprivation can be highly selective.Average rates of intracellular protein degradation are enhanced in cultured cells that are deprived of serum, hormones, growth factors, or nutrients (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). This increased proteolysis is physiologically important because amino acids are used as an energy source under deprivation conditions (16,17). It is now generally accepted that lysosomes are responsible for most of the enhanced degradation during withdrawal of serum, insulin, and amino acids (6,14,17,18). However, the mechanisms by which cell proteins are delivered to lysosomes and whether such a process might be selective for certain cell proteins are still unresolved issues (14,(18)(19)(20)(21)(22)(23).The increased proteolysis during serum or hormone withdrawal appears to be somewhat selective in that it applies to long-lived proteins only. Short-lived proteins are affected little, if at all (2,4,6,14,24,25). Degradation of two different molecular classes of long-lived proteins (small and basic) is increased during serum deprivation (26). We originally interpreted similar findings in tissues of diabetic and starved rats (27) as support for autophagy being a relatively nonspecific process superimposed on a more selective "basal" proteolysis (6,27). Further consideration of our results (28) combined with additional data (25,29) show that autophagy must, in fact, be selective for the more long-lived, small, and basic proteins. These results imply that deprived cells are able to recognize certain cellular proteins for enhanced degradation while sparing others.To probe in more detail the mechanisms of selectivity in enhanced proteolysis, we have examined the degradation of bovine pancreatic RNase A (RNase A) using erythrocyte-mediated microinjection (30,31 Typically, 0.5 ml of protein solution (20 mg/ml in 0.2 M borate buffer, pH 8.9) was mixed with sufficient 0.2 M formaldehyde to be in 4-fold molar excess of protein primary amino groups. To...

Intermittent compressive (IC) forces (96 mm Hg, 0.3 Hz) inhibit by 35-60% the serum stimulated increase in ornithine decarboxylase activity (ODC) in chick embryo epiphyseal cartilage cells and rat chondrosarcoma cells. IC had no effect on mouse fibroblast L-cells ODC. The dose-response pattern of the IC effect indicated an all-or-none response with a threshold a t 80 mm Hg, a pressure roughly equivalent to the in vivo weight bearing force. The k, of the cartilage cell ODC, measured a t four hours, was about 0.1 mM and was not affected by IC. The Vma, on the other hand, was significantly reduced by IC which is consistent with less enzyme or non-competitive inhibition. IC also produced a significant increase in cAMP levels in both cartilage explants and isolated cells in the presence and absence of serum and a significant reduction in 3H-thymidine incorporation into DNA. The findings show that cellular CAMP, on one hand, and ODC and DNA synthesis, on the other hand, change in opposite directions following exposure to serum and/or IC. Investigation of the IC effect on DNA synthesis in serum-deprived synchronized cartilage cells revealed that IC reduced the number of cells going into S but did not lengthen the GI phase. Exposure to IC early in GI (0-13 hours) produced the full effect, whereas IC application between 13 to 24 hours (pre S) had no effect. IC had no effect on 3H-thymidine incorporation in L-cells.It is well established that mechanical forces play an important role in bone remodeling (Wolff, 1892). In attempts to elucidate the mechanism for the translation of the mechanical stimulus into a cellular signal, Bourret and Rodan ('76) and Rodan et al. (' 75b) have exposed chick epiphyseal cartilage and cartilage cells to low magnitude (0.15 at m) continuous compressive forces and have demonstrated reductions in cAMP levels, apparently produced by increased calcium uptake. The continuous force used in these studies simulated interstitial pressure, muscle tonus and weight bearing at rest when the proliferative activity in the skeleton is at its peak (Simmons, '64). During walking, running or hopping, however, the epiphyses are exposed to intermittent compression, which may provide a different physiological stimulus than continuous force. It was shown, for example, that the initial response to repetitive articular trauma is enhanced matrix production and the absence of cell prolifer- (Muir, '77). An inverse relationship between the two processes is also seen during the maturation of epiphyseal cartilage cells. It was therefore of interest to examine (i) if intermittent compression (IC) elicits in epiphyses of tibia explants a different cAMP response than continuous pressure, (ii) if cells isolated from the epiphyses can detect IC in vitro, and (iii) if IC has an effect on cell proliferation. Two parameters related to proliferation were investigated: 3H-thymidine incorporation into DNA and ornithine decarboxylase (ODC) induction. ODC controls the rate limiting step in the synthesis of putrescine (Pegg and...

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