Direct inhibitory actions of GnRH on accessory reproductive organs of rat

Immunotherapy of prostate cancer in a murine model using a novel GnRH based vaccine candidate

Previous studies with gonadotrophin releasing hormone (GnRH/LHRH) vaccines have shown the usefulness of immunization against this hormone in prostate cancer. To this end, we have generated a completely synthetic peptide modified at position 6 and attached to the 830-844 tetanic toxoid (TT) helper T cell sequence. Through this work we have demonstrated that the GnRHm1-TT molecule was highly immunogenic when it is formulated as an oil-based emulsion adjuvated with Montanide ISA 51. That results correlated directly with testosterone reduction and tumor growth inhibition of the Dunning R3327-H androgen responsive prostate tumor model in rats. GnRHm1-TT, proved to be safe and useful for future clinical trials.

Synthetic luteinizing hormone releasing hormone (LHRH) vaccine for effective androgen deprivation and its application to prostate cancer immunotherapy

We have designed a peptide-based immunotherapeutic vaccine for treatment of androgen-responsive prostate cancer. The vaccine targets the luteinizing hormone-releasing hormone (LHRH) decapeptide that results in an androgen-deprivation immunotherapy. The design elements of the peptide immunogens are the LHRH peptide or B cell epitope synthetically linked to different promiscuous helper T cell (Th) sequences, the UBITh epitopes, derived from four natural pathogens for effective immunogenicity in outbred populations, and in some cases, also linked to an adjuvanting peptide from Yersinia invasin (Inv) protein. The UBITh LHRH immunogens are adsorbed on Alhydrogel or formulated as several different oil-based emulsions and tested in rodents, dogs, and a non-human primate, baboons. The immunogens generate an anti-LHRH antibody response specific to the LHRH decapeptide element in contrast to LHRH conjugate-carrier protein vaccines where only a small portion of the antibody response is directed to the target epitope and epitopic suppression is noted. Individual UBITh peptide domains, but not the LHRH and Inv peptide domains, are stimulatory in lymphocyte cultures. The UBITh LHRH immunogens in a clinically applicable formulation, controlled the growth of Dunning R3327-H androgen-responsive prostate tumor cells in rats. The results demonstrate universal responsiveness and long duration of androgen deprivation from three diverse species, and thus vaccine efficacy.

Effect of active immunization against luteinizing hormone-releasing hormone on the androgen-sensitive Dunning R3327-PAP and androgen-independent Dunning R3327-AT2.1 prostate cancer sublines

BACKGROUND

The objective of this study was to determine the effect of active immunization against LHRH on the growth characteristics and histology of subcutaneously implanted tumors of the androgen-sensitive Dunning R3327-PAP and androgen-independent R3327-AT2.1 rat prostate adenocarcinoma sublines.

RESULTS

We herein demonstrate that 1) active immunization with an LHRH-diphtheria toxoid-conjugate (LHRH-DT) leads to the downregulation of gonadotropins and testosterone and consequently the atrophy of testosterone-dependent organs such as the testes, prostate, and androgen-sensitive Dunning R3327-PAP tumors, 2) growth inhibition of Dunning R3327-PAP tumors is caused by suppression of cell division rather than by an increase in cell death and is associated with an increase of the tumor stroma content, and 3) volume increase of the androgen-independent Dunning R3327-AT2.1 tumor is slightly but significantly reduced, indicating a local stimulatory LHRH loop within this tumor cell line.

The effect of active immunization against gonadotropin-releasing hormone on the ultrastructure of the rat ventral prostate

To evaluate the effects of active immunization against gonadotropin-releasing hormone (GnRH) on the ultrastructure of the rat ventral prostate, male Sprague-Dawley rats received three consecutive intramuscular injections of 10 micrograms/100 g body weight (D-Lys6)-GnRH-diphtheria toxoid conjugate (GnRH-DT vaccine). Following immunization, test animals developed sufficiently high antibody titres to block the pituitary gonadal axis. Consequently testosterone values dropped to the levels in castrates. This therapy leads to atrophy of the prostate. Following immunization a strong immunological response, indicating the presence of considerable amounts of a GnRH-like peptide, was observed in the ventral prostates as early as 14 days after the first injection of GnRH-DT. Immunoneutralisation of GnRH-like activity may contribute to the effects observed.

Gonadotrophin-releasing hormone: physiological significance and relevance to cancer

Gonadotrophin releasing hormone (GnRH) is a decapeptide released by the hypothalamus. The binding of the peptide to pituitary receptors leads to the activation of second messenger systems. The physiological outcome of the exposure of pituitary cells to GnRH is the release of luteinising hormone (LH) and follicle-stimulating hormone (FSH). Continued exposure of these receptors to high concentrations of the peptide desensitises the receptor, thus inhibiting the release of gonadotrophins. This paradoxical effect has proved to be beneficial in the clinic where long-acting and enzyme-resistant analogues are used to inhibit the pituitary-gonadal axis, for example in the treatment of advanced prostatic cancer. In addition GnRH-analogues may affect tumour cells directly as observed in vitro. These direct effects have been described as inhibitory but recent data suggests that low concentrations of GnRH-analogues may stimulate short term growth of prostatic cancer cells in vitro. GnRH shares many other common characteristics with peptide growth factors, including common second messenger systems and receptor desensitisation on prolonged exposure to the ligand. It is possible that the direct inhibitory effects of GnRH-analogues are mediated through the desensitisation of tumour GnRH receptors, as suggested by recent observations. The nature and mechanism of the direct anti-tumour effect is important to understand and to promote the therapeutic efficacy of GnRH-analogues in the clinic.